I am sure it was 80 thou as the cooper & MG 1100 had D top pistons It was common to break in to the drilling for the rocker oil feed Araldite and bundy brake tube was the repair12G295 was used on 998 Coopers and MG 1100 too -the combustion chamber volume on these and the 12G206 single carb version is bloody enormous, in order to get a decent compression, they needed a huge amount milling off the head face if you weren't using proper Cooper pistons.
12G940 with 9 studs was the run of the mill 1300 head.
Early Cooper S's had a different head which I can't remember the number of.
Mk 3 Cooper S cars did use the 12G940 head, but the "S" version had the extra stud holes drilled at each end of the head and bigger valves than the "standard" 1300 version.
I new the 185 was a good number the 1100 would not need the D pistins with the longer stroke (it has been a long time )lolThe 998 cooper had the D top pistons, the MG1100 didn't, in fact the cooper was the only type to use these pistons. i.e.. non of the other engines that used the 295, 206 had these.
Head stud wise, all cooper S used 10 studs and one bolt.
The early S head was the 12A185 and these are extremely rare now a days, most S's had the AEG163 head, which was later replaced with the 12G940 (1805) casing. As above, these earlier heads where very prone to cracking.