Those measurements are a bit odd, if I made my math correctly that is aprox a 55 mm caliber. Are there any markings in the base?

I say odd because the closest allied caliber was 57 mm, mainly an anti-tank gun and a more rare recoilless rifle.

Going with the assumption that it is a 57 mm caliber then the M1 gun is the most likely weapon to have used this. It was a British gun and Brits tended to go by the weight of the projectile instead of its caliber, so this would then be a 6 pounder.

This picture resembles yours, with case...

https://upload.wikimedia.org/wikipedia/commons/9/9f/57mm_Gun_M1_-_Saint-Malon_Bretagne.jpg

This would be the gun, in service with both US and British Commonwealth armies.

https://en.wikipedia.org/wiki/Ordnance_QF_6-pounder

There was also the M18 recoilless rifle but this was is unlikely.

Axis forces had nothing similar in this caliber, the closest would be 50 mm and 47 mm for ani-tank purposes but they would not fit the measures you provide.

So assuming my conversion from inches to mm is correct the M1 / 6 Pounder is the most likely candidate.

might want to try /r/whatisthisthing