Christoph,
Have a look (if you have not already done so) at "Gaining a RAF Pilots' Brevet in WWII" (a few doors down at the moment, widely regarded by many as the Best Thread on PPRuNe). Plenty of old-timers' tales there.
One reason for the paucity of 20mm and 0.50 (these would be mostly American) cases might be that they are bigger, and therefore more easily fashioned into ornaments etc, than the smaller stuff. And the scrap brass collectors could get a decent load without so much picking up to do.
So Indian 0.303s turned up (how would you know ? - I can't remember if there were any code markings on the base to give a clue, and I handled thousands of them). If they did, probably from Dum-Dum (Calcutta). Small world !
Cheers, Danny.