Absolutely not. Your ratios are correct. I think you have a 1.5 litre gear lever though.
Here are a few pics of 2.5 and 3.5 litre SS Jaguars. The levers are much straighter than in the 1.5 MkIV.
In neutral the lever should be almost vertical.
Peter
Hi Peter and Rob,
Yes, I was confused because the gear lever set up is per 1.5 in the Mk IV, which is more what I was familiar with. You see why I say I’m an experienced ignoramus when it comes to trying to know the numerous subtleties between pre and post war models.
Just a comment on your maths with the ratios, Rob. You actually don’t need to bring the idler into the calculation because it is only there to change direction, i.e. one tooth movement of the laygear enables just one tooth movement on the main gear through the idler and it doesn’t matter how many teeth it has. The 21 cancels itself out in the formula as it is both numerator and denominator. We love maths don’t we. That’s what my teacher said anyway.
Relax Rob ! you have the correct box.
I have broken for parts pre war cars with straight and cranked gear levers.
The only gearbox ident numbers I have
seen are the “BREVET” numbers on the top cover.
Peter B.
Thanks to all three Peters participating in this discussion, all of you in the UK I believe, and Roger, all very much appreciated! And glad you put your full names so I can keep track. I must admit I am less inclined to those who use a pseudonym or mis-identifiers.
Had to look up that word Brevet, it means raised in rank (of an officer), so now I understand.
A couple of patent numbers in the 400,000 range and something I can’t make out with a seven digit number.
It would be interesting to see the original patents if I could make out those numbers. They can be found here.
https://worldwide.espacenet.com/?locale=en_EP
I did the reverse ratio calculation just to satisfy myself that I hadn’t forgotten how to do those.
In addition to the ratios given in the 1946-48 Service Manual, which indicate on page 14 that I have the ratios for 2-1/2 and 3-1/2 (with a misprint for 2nd) but not 1-1/2 cars; I found this page 6 in the 1938 Instruction Book.
Now I wonder when the ratios changed to those found in the '46-'48 book?
Does anyone have information on ratios for '39 and '40?
BTW I think I’ll keep my cranked shift lever, as I found it very comfortable to drive with. In fact after putting about 100 miles on it, I am even somewhat comfortable running a RHD car in a right side driving country. 
Rob the Standard Mo Co used commision numbers not chassis numbers, and this carried over to early SS models pre OHV days, I think that Brevet is used in this commision sense.
English but living in France I RHD in a RHD country, it enables one to hug the kerb.
I will add one of the few things I can hug these days.
Peter B.
Sorry Rob, I don’t have a clearer photo of my gearbox top.
I think the 2-1/2 litre ratios that you have calculated from the 1938 handbook are what my car has and your car had originally but from the parts list I think the box in your car is from a car with engine number L1451 to L2055 corresponding with your double helical with needles.
My spare gearbox is not very accessible and only the first patent number makes any sense.
Reading your second number and mine makes sense.
Wow, I love this forum.
So Brevet was actually stamped on there. Perhaps its like a brevet officer receives a commission.
Patent GB407576 belonging to the Standard Motor Co. appears to be about the synchromesh on 2nd gear, and introducing our beloved plunger and spring loaded balls.
Patent GB427363 also belonging to Standard Motor seems to be about the double synchromesh on 3rd and 4th gears. Only four spring loaded balls. Mine has six.
The third number looks like Drawing No. 333923 which I suppose was in the Standard Motor Co. engineering office, perhaps the final assembly drawing.
For Peter Scott’s third patent number, I tried guessing a sixth digit 0-9 but none of them were related to gears.
I imagine this means Standard built the boxes themselves for awhile, and then turned the job over to Moss Gear in Birmingham? Or somebody called SGK?
Interesting history of Moss Gear on this web site.
https://www.gracesguide.co.uk/Moss_Gear_Co
Rob, Looking at the parts list I see your synchro balls & springs are the same size as the later single helical which means you probably don’t have to guess the weights like me and have them made specially.
Peter
Do you mean the C922 balls for the synchro sleeves, or the C923 balls for the shifter rods? I noticed when you wrote that, and was dismayed because I figured they would be some standard inch size like 1/4" or 5/16" and readily available here. Maybe they are rare in a metric country now.
I have a guess as to why there is a flat cut on the tail end of the counter shaft.
I wonder what car this one is for?
A fair bet is a 16 or 20 hp Standard.
Rob, It was the C.900 springs I had in mind. My springs a 1/4" if memory serves me right and yours 5/16".
Your thoughts on the layshaft flats sound very credible and I think Peter B. is probably answering your Moss advert question.
See also: Meet the Cousins
Peter S.
Peter L’s comment may be relevant. If we are correct in thinking that your gearbox came from a car with engine number between L1451 and L2055 then perhaps the 1549 number on the top cover was originally mated to engine L1549. My engine is actually older than yours Rob but when it left the factory in January 1939 my car it had L1424 so the transition from double helical with bushes to DH with needles took place at about the 1938/1939 year change and I think the change to single helical took place early in the MkIV just to give you a rough estimate of the date of your box.
Peter
I thought some boxes were assembled by Moss , and some by Jaguar and to identify which, some had a small groove marked around the centre of the teeth
Early [ very] gearbox es had the double helical teeth overlap a little in the centre, later had a deep groove separating them
Probably for easier manufacture
I wonder if you’ve got these the wrong way around Ed? The photos in the 1936 Owners Handbooks show the non-overlapping deep groove gears and given the needle rollers and gear ratios of Rob’s box with its overlapping teeth it appears to correspond with the boxes fitted between engine L1451 and L2055.
Also the patent drawings from 1933 show the non-overlapping deep groove cogs.
Peter
Ed, you are quite right that Jaguar did make gearboxes, but I believe it only began with the JH single helicals about 1948. Happy to be proved wrong of course.
Gears certainly are a difficult thing to manufacture, and I wonder how they made these with double helical teeth and synchronized teeth together.
My engine is L205 but it is looking like that probably has no relation to the number 1549 on my top cover.
Peter S, you wrote that the needles were in boxes from engines L1451-L2055, and I did not want to contradict you, as I thought the needles only appeared in engines 1-17 and P18 and subsequent, but maybe our sources of information differ. Here is what I find in my '38-46 parts book.
All this is relevant because I need to figure out if I can use the counter shaft and needle bearings for SH and JH boxes in this one.
Mine measures .980" diameter in the unworn places. I heard from XKs Unlimited and the salesman said he measured one as .990" but he may or may not be familiar with the use of a micrometer.
My apologies Rob,
The drawing above contains the “J” parts listed as for engines L1451 to L2055 and it also shows the needle rollers but I stupidly didn’t check which engines used part J44.
So it looks as if your gearbox came from one of the cars fitted with these engines:
or with engine numbers P18 to P899, after which the single helical box replaced the double helical and the top cover changed from C876 to C1927 that has the long lever travel. I think that change took place sometime in 1947.
Peter
Looking up the XK120 parts lists the LH, JH and JL boxes they all use 58 needles C918. Unfortunately the actual layshaft and thrust washers are not the same parts specified for your box but I’d have thought it’s a fair bet that the shaft is the same diameter if nothing else.
Peter
Everybody has a brilliance day, or a why didn’t I think of that before day. Today is mine. I have a broken SH box here, so I removed the locking plate and tapped the counter shaft forward 1/2" so it came out the front end where I could measure it.
It is .980" diameter and 11-1/8" long, same as the counter shaft in this double helical box, so I am ordering a C1856 counter shaft and C38573 needle bearings from XKs Unlimited.
Huw! Far too easy. You at least should have had to cut 1/2" off the length or grind a different flat on the end. 
Sounds like you are on the way to a good repair.
Peter 
Rob,
Are you saying that single helix boxes were fitted from 1946? My 1947 3 1/2 litre has a double helix box.
Art
