If I recall correctly from discussions on Tanknet, the tungsten penetrator was always in short supply. Ammunition issues seem to be the curse of the allies, The Boyes ATR only got a decent round after the gun was more or less obsolete, the 2 pdr was a good AT gun, but they never bothered to make a HE round until late into the war. The 6pdr with APDS was quite deadly and could penetrate more armour than the 75mm, but due to size had a poor HE performance, plus the APDS did come out till around early 44 if I recall correctly. The 17pdr had an issue with the APDS that lowered the accuracy that took some time to deal with. The Brits had a deadly AT gun in the 32pdr but never bothered to develop it into a tank/AT gun until they mounted it into the Tortoise (Post war tests on German armour was quite impressive, ripping apart a Panther’s front Glacis plate).
The 75mm in the Grant and early Sherman’s was based on the French 75 and issued with old stocks of ammunition. I seem to remember the Brits modifying captured German Stocks of 75mm to fit their 75’s.