Threaded Core Plugs in the Valley

Curious to know if this has been researched.

This is the type of core plug in the head of my XK120, part number C.2157, also used in Mark VII and XK140.

This is the type of core plug used in a Series 2 E-Type, also listed as part number C.2157 and also used in XJ6.

The thread is the same 7/8-14 UNF for each.
Its kind of odd that they didn’t change the part number, not even a /1 suffix.
My question is this:
When did they change from the internal hex key plug to the external hex head plug?

I believe with the advent of the B series head in the late 1950s.

I dont have extensive data/photos, but the few I have suggests not from the start of the first variant C12500 B-type head (as fitted to the very first XK150 3.4, Mark VIII and 3.4 litre (1957 onwards), still fitted with ‘hex-key round plugs’ up to at least this HSN of CJ428


But the later type of B-type head, the revised C.14958 B-type head as fitted to 3.8 XK150 and 3.8 Mark IX and 3.8 Mark 2 from 1959 on , from at least HSN EC73 onwards, does now have the Hexagonal type plugs…

Now there was also a later 3.4 B-type head C.14956 introduced about the same time, also with Hexagonal Plugs, but I suspect the HSN gap between CJ428 and EC73 will be the demarcation point, but possibly exactly at the C.12500 to C.14956/ C.14958 change point which is quoted to exact Chassis Numbers.

Thanks Mike and Roger, good information is always appreciated.
So between CJ428 and EC73, if they used all the Dx letters except DI and DO, and if my calculations are somewhere close to accurate, the demarcation is somewhere within about 41,600 heads.
You can see why they made the change, the hex head would be cheaper to make, and easier to remove if necessary. I had a bear of a time getting a couple of my socket head plugs out.
Funny they didn’t change the part number though.

Note that the same part number is used for the big hex head plug at the back of the main oil gallery on every XK engine I can recall (I’m not familiar with the earliest versions).

All I can currently find for 7/8” 14 tpi Allen head plugs are o-ring hydraulic pipe plugs. These have a far smaller circular flange than the OEM copper washer version and look all wrong. Have tried Welsh, SNGB and C-W so far…