Hello Chris:
I checked
Janes All the World's Aircraft 1945-46, which was issued in Feb 46. Noted the following:
- Spitfire Marks are designated with Roman numerals to Spitfire XX, thereafter Spitfire 21 and 22
- UK production Mosquitos with Roman numerals to Mosquito N.F. Mk. XIX, thereafter Mosquito N.F. Mk. 31, but also note Canadian production with Roman numerals to Mosquito B. Mk. XX, thereafter Mosquito F.B. Mk. 21
- Beaufighter went from Beaufighter XII to Beaufighter 21 (Australian production), no Mks between XII and 21
- I also checked designation of the Rolls-Royce Merlin engine, it went from Merlin XX to Merlin 21
It looks as if it could have been Air Ministry policy to use Roman numerals to XIX or XX, thereafter 20, 21.....
This is definitely outside my usual sphere, but at least I hope the above gets things started.
Cheers
Don (Boomerang)