Sorry, I'm not smart enough to give you definitive information. However, I did search for a while.
From the Mk1 Mini Forum:
https://mk1-forum.net/viewtopic.php?f=3&t=4902&p=34140&hilit=12g940+heads#p34140
The numbers after the casting mark are in inches/mm for the intake... then exhaust.
Only one of the later 12G940s in the list below mentions special marks/stamps. I think for most you need to measure the valve diameters. Threads I found while searching suggest there were at least 7 or 8 different raw castings for the 12G940.
My Mini was built with the AEG163 head but like many of those castings the valves had sunk into the head. Though my car's head was not cracked, in general they are prone to cracking between the inlets and exhausts. I replaced my car's head with a large valve 12G940 and have been happy. However, I didn't know about the physical appearance differences. The head I bought was from an A+ engine so the head is not perfectly flat around the thermostat area (see the text below the list below).
"AEG163 1.401"/35.6 1.22"/31.0 21.4 Later Mk1 & Mk2 Cooper S
12G940* 1.312"/33.33 1.15"/29.2 21.4 1275GT, Austin 1300, all late A+ models inc. Turbo but not MG Metro
12G940 1.401"/35.6 1.15"/29.2 21.4
Stamped '12G1805' on flat area by thermostat - MG1300, 1300GT & Mk3 Cooper S
12G940* 1.401"/35.6 1.15"/29.2 21.4 As above plus MG Metro
*Note:
A+ head castings - despite having the same 12G940 casting number - are considerably different in appearance, loosing the flat area behind the thermostat housing and around the rest of the rocker/head stud areas to a sculptured look - where these areas are replaced by a 'sunken' cast finish.
The A+ castings were introduced with the appearance of the Metro in 1980, although some late 1275GTs (1979-on) had them fitted. Generally they are easily recognised by their colour - a putrid yellow on the small-bore castings and bright red on the large-bore ones, a paint finish that is VERY difficult to remove, even in a chemical tank.
The MG Metro Turbo shares the same valve sizes with the other non-'big valve' 12G940 variants but has 8mm, sodium-filled exhaust valves.
A+ heads generally have three collet grooves in the valves, collets are 14-degree cone taper instead of the 10-degree on single groove collets (so parts are not interchangeable between single and triple groove types) and the top-caps have a raised ridge around the collet hole."