According to the 1939 parts book, all models were fitted with the same gearchange springs 66-3224 and 66-3225. They were changed for 1940 and the earlier ones appear to be no longer available. The shorter one on my B21 is incorrect and cobbled together to provide tension. Does anyone know a source for these springs or failing that, what the dimensions of the short spring would be? I'm guessing around 47mm long, 8mm diameter and 7mm end loops.
Cheers
Paul
After more digging I've found that most of the various gearchange parts for the B21, M20 and B33 etc have the same part numbers, apart from these elusive springs fitted in 1939. I couldn't find the part numbers for the B21 pawls. It doesn't make sense. I've photographed the stripped gearchange to see if any eagle eyed viewer can see anything wrong. The only suspect area might be the reprofiled pawl ends. Mine come to a point whereas pictures online show them as rounded.
My 1937 BSA Y13 has the same 4 speed foot change gearbox and the same springs. Yours don't look too bad. I'm sorry but I'm not able to take the cover off my gearbox to measure the springs (much too old). As you say they are unobtainable so the only solution would be to fit modern springs that could do the job, Having new springs made would be very expensive and you would need to find others to share the cost of a small batch. Best of luck.