Mogwi ...
I don't think I'm going to be of much help here, but the following might help a little with your quest
I believe at Linton from 1961 to 1969 the Chipmunk fleet was coded A to Z but I don't have the X-Ref to airframe Serial Numbers. However, I believe (but this will need substantiating) that WP809 was coded 'S' before she later displayed '912' as a RN Cowling Code.
It's quite possible that the other RN Chipmunks, with the following 3 digit Cowling Codes, were also pre coded with 'Alpha Codes' ... but again I don't have detail on the X-Ref to airframe Serial Numbers.
WP795 901
WK634 902
WD374 903
WP856 904
WK511 905
WK608 906
WB575 907
WB657 908
WP904 909
WB671 910
WP801 911
WP809 912
Hopefully Bill might have better luck ... Equally we have a few other RN contributors to this thread who might be able to help.
Best ...
Coff.