If chromed versus polished, is it the more common chromed diecast alloy instead?
Peter, my question relates to the relative processes, as apparent (to me) in Robs photosā¦
With Chromium Plating, the bright colouring is the Nickel underlay, not the colourless but hard top chromium layer. But the brightness of the Nickel/Chrome is totally dependent on the surfcar preperation of the metal to be plated. So on highly polished steel, brass, or indeed cast aluminium as these bases are, you can get the bright exterior surface of Robās pictured base. But if the Nickel (and Chrome) are applied to a non-polished/prepared surface, you get what is called Dull Nickel or Dull Chrome - as per SU Carburetter unpolished brass lever armsā¦
So this cast aluminium base - given that Nickel and Chromium plating is applied in an emersion chemical Nickel bath, you would get Nickel applied to all the rough internal areas of casting, and also the smooth but not polished base, as well as the highly polished outer surface. I canāt see any signs of dull Nickel (Chromium) Plating on the inside and base of the casting in Robs photos, thus I asked for Robs confirmation, as what he can see in hand, given he does have it in hand, and Rob knows what he is looking forā¦ With cast aluminium, even on the highly polished surfaces, you can have some porosity/imperfections, so again in photos itās hard to say whether the imperfections are in the aluminium or indeed failure/ageing of any Nickel/Chrome platingā¦ Rob will be able to determineā¦
Great information and detective work - thank you Roger and Rob.
I have rechecked my old photos and the āholeā in the original tool box base was not a defined rectangular or oval shape, so I conclude my more rounded lamp had probably been replaced before I got the car, and the car propably had a different lamp fitting before that.
My old interior had woodworm, and therefore replaced. The new āholeā in my new tool tray base was made round to fit my round lamp!!
Well, it sure looks like chrome to me. Note the reflection of my phone and fingers. I wouldnāt think you could get that kind of reflection from 85 year old polished aluminum.
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Nothing to do with the original posterās MkIV question but my SS has black mounted on a chrome base.
Peter
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More pix.
This base is heavy, so I think it is not aluminum, it is cast brass which has been chromed. The periphery (visible outer surface) was probably polished first before the chrome was added. It cleaned up pretty well with fine steel wool.
The little oval cover plate is steel painted black as Peterās.
Good to get these confirmations, as well as the black padding on the lid; mine is the same, not green felt as on later cars.
I have ordered the MGTA map light cover, so Iāll let you all know if it fits, or if Iāve wasted $71.
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If itās heavy and the pitting is just spots and not erosion, itās brass/bronze and not aluminium or die cast alloy. You can confirm further by filing a little from the edge of the boss around the hole on the underside to check the colour.
The cover used for the MG lamps will be much better than none as it protects the bulb from damage, and its surroundings from heat when you close the lid. The operational difference will be the spread of the beam, which will be narrower front to back because of the cut-off at the outer end, compared with the original cap.
Confirmed, itās a brass casting, chromed. I can also see that my bulb is too large.
The tool tray light in my 1939 saloon looks much like Peterās. I think the base surround is chrome, not aluminium. The bulb base rotates to switch it on, and it is wired through the sidelight circuit.
My padding has been green at least since 1968!
Denis
Well done Rob - as before, you really need an original example in hand, with someone who understands metallurgy looking at itemā¦ As before, I could not reconcile this item being CHROME (or just NICKEL) plated if it was cast aluminium, as the visible rough interior looked to be cast aluminium (in particular) and the bottom surface looked to be as machined aluminium, in the photos then provided, thus my comments about Bright versus Dull Nickel (Chrome) plating. And if that was the case, then no one is going to polish and plate just the outer curved surface, with masking up the bottom/rough cast interior. Now Rob has confirmed its cast brass, not cast aluminium, that explains the Dull Nickel (Chrome) plated appearance of the rough interior casting and the unpolished bottom surfaceā¦ I still question whether it is just Nickel plated, or has the added clear micro layer of Chrome added on top of the Nickel, purely because generally in UK, in the 1930s Nickel plating was more prevalent than the extra Chrome Plating for ornamental items, but probably dependent on who made/supplied these bases, and nothing to say they didnāt change over 1937MY to 1939MY period of useā¦ But itās not easy to differentiate Chrome plating from just Nickel plating even on new items, let alone 85 year old items.
Refer to my earlier photo, a superb and extremely original 1937MY car, but actually built in Nov 1936, so one of the very earliest examples of the newly introduced for 1937MY Tool Tray Lampā¦
So, now assuming oval base is cast brass as now established, not cast aluminium, its dull perimeter appearance suggests no polishing, so just Dull Nickel (Chrome) plating. The actual Lamp - I still think is a Lucas DF41T (Model) is another example where the oval platform (base of Lamp) clearly a polished-BLACK finish - I would say most likely āJapan Enameledā, and secured by two chrome-plated slotted head Raised-Countersunk headed Wood Screws (presumably, being typical onto timber frames, rather than set-screws, and as per Robs photo) - and in the 1930s, not Phillips Head of courseā¦ Might be a difficult fastener to source these days, if originals are missingā¦
I forgot to mention the brass was confirmed by the Peter L scratch test. 
I certainly do not claim that my flat head slotted screws are original. What was in there was raised head slotted screws as Roger said, but they were half rusted away and not really capable of holding anything, so I replaced with the closest thing I could find.
And thanks Denis for letting me know how to operate it. Quite right, turning it clockwise turns the light on, and counterclockwise is off. It actually pops in and out a bit when you turn it.
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If you look at the side view in the earlier photo above, youāll see the oblique slot which guides the contact disc. The disc moves up and down with its attached live wire, making and breaking contact with the bulb. If the guide pins drag a bit in the slots, a little vaseline helps.
I think you may have a fault with the assembly, as the rotating bulb holder has to be firmly attached to the bottom of the black base unit. The bottom of the bulb holder is swaged over to grip the base but it is free enough to allow rotation for switching. If yours is not fitted together properly, you may have to replace it. This feature is important as it is the route for the earth. The MGās dashboard is timber and the earth continuity is made by one of the base unit screws being a metal thread right through the dash with washers and nuts on the back. If your unit is fixed to timber only, there needs to be an auxiliary means for earthing.
There are two features out of view in your photo. 90Ā° from the join you can see in the base spigot, there are two slots for engaging the pins; and around the āfloretteā (I donāt know what else to call it) there will be a wider space between the fingers which is the keyway to engage the cap.
Iāll stop rambling now.
Peter L
Yes, I see that slot in the florette. Thatās a good name for it, so I think weāll stick with it. Iāll let you know when the MG cover arrives and how it fits.
Mine has a ring terminal end with a wire soldered on the tail end of the socket as a ground (earth). The other wire passes through the center ring hole.
I was thinking about why they had this chromed brass base. Itās because it is on a tool lid rather than a dashboard as on MG. The tail end of the socket is too long and would protrude out of the 3/8" thick plywood lid, so they had to set it higher up, and provide a large enough clearance hole in the tool tray. The later type light didnāt have that problem.
Thanks for the advice and info here - most helpful. Iāve managed to adapt a courtesy light switch which works. However, Iāve now a second problem - an intermittent contact on the back of the bulb holder in the tool tray lid, or possible a fracturing wire going up to the holder. My question is - how does the tool tray lid come apart? Itās not obvious!
First you lift off the green padding, which is glued down.
Then you will see a channel cut in the plywood, and the wire feeds through it under some sticky tape.
It may be possible to feed new wire through without pulling up the green padding. Use the old wire to pull the new wire through.
If you need to remove the panel, firstly remove the rubber inserts in the rubbing strips, then the set of wood screws which are screwed into the plywood backing panel. This releases the panel completely.
For the wiring, you can just pull them down as covered by Robās description, without disturbing the baize lining. I found that the wires fed through easily without a draw string, as mine came adrift once, so I donāt use one any more.
Ah, thatās most helpful - Iād assumed the wiring was behind the wood. Being in a groove should make things much easier. Thanks.
Thanks again to all - responses to this post have been so helpful.