I think John is wondering a little bit too much about your Hotchkiss rebuilt.
It is was a Mars ctge or something very rare I would understand.
But a 37 mm like that is very common. about 10 $ a complete one in France on any flea market.
On the other hand you are right, most of the big shells are rebuilt : case found in one location, round coming from another place.
The medium caliber (20 or 30 mm) are usually found complete, at least in France.