[Juha]

Hello Bill
to the end of 1942 roman numbers, then new variants in Arabic numbers, from 1948 all a/c variants with Arabic according to Alfred Price. I also recall a 1943 letter from 5th Sea Lord to Air Ministry, which is quoted in Morgan’s & Shacklady’s Spitfire book, in which the Sea Lord used designation F.21, when talking on newest version of Spits in test-phase. He wanted a naval version of it as soon as possible, even if that meant non-folding wings. But it clearly isn't quite clear-cut. Price begin to use Arabic on Mk 18, M & S on F. 21.

Juha