[Larry]

So I understand only 30 cannon armed Spitfire 1bs were made/ modified and doesn't help that they were later updated to Mark V

These are the known Spitfire Mk Ib with cannons that I can find so far;

R6761, R6770, R6776, R6778
R6809, R6833, R6888, R6889,
R6890, R6897, R6904, R6908,
R6911, R6917, R6919, R6923,
R6958

Most of these started out with 19 Sqn and R6923 also moved to 92 Sqn.

All we need now is the other 13 serial numbers.