Originally Posted by
Davef68
No, the T11 was the next progression in the Vampire family (FB9, NF10, T11)
The Chipmunk was the T10 due to a short lived policy in the Air Ministry to give mark numbers in blocks - the actual reason is not known to me, but another example was the HP Marathon which was given the T11 designator. I've heard it explained as 'military adaptions of civil aircraft' or 'trainers'.
A longer lived variation was using the '20' range for Naval adaptions e.g. Vampire NF20, Sea Venom NF20/21/22, Sea Devon C20, Sea heron
In the case of the Chipmunk, Mk.1 to Mk.9 were reserved for the RCAF variants, in the days when the RCAF still used British-style mark numbers, and Mk.20 onwards for export/civilian versions.