I've been having a look through Austin Service Journal - War Department Issue, sadly I only have a couple of issues (Vol.9 commencing January 1945 and Vol.10 commencing January 1946). There's a note in Volume 9 that says:
Further to the article appearing on page 98, Volume 8, of this Journal, the engines of 4x2 vehicles first fitted with the modified compression and scraper rings, part numbers 1K1420 and 1K1421 respectively, are as follows:-
2-Ton W.D. Ambulances, engine no. 75090, during Contract S.3165
2-Ton (4x2) R.A.F. vehicles, engine no. 71804, during Contract S.6566
That doesn't advance things too much other than to confirm there was a design change. Presumably engines fitted to other types in the Austin range were also modified - you'll need to compare and contrast parts lists and/or track down Volume 8 for more.