I wonder if this was the case for
Oerlikon's 20 mm cannon and
Bofor's 40 mm cannon that seemed to be used by many nations.
Standardisation between nations is
HARD. Look at the case of the Hispano-Suiza HS.404.
The UK and US both purchased manufacturing licenses for it (the US for an obscene amount more), so theoretically their guns could have been interchangable. But, both countries modified the original drawings to their own standards and there were issues with the US built versions (chamber too long, weak single recoil spring). As a result, a common production standard was never reached.
From Chin's
'The Machine Gun' pp 556-557:
"... promising reports [about the HS.404] were coming in daily from the British, who had used practically the same procedure as our Army ordnance engineers in getting the gun into production status. The original Birkigt Type 404 gun was used as the model for the Mark I. This was followed shortly by the Mark II.
Drawings and a sample of this improved cannon arrived in the United States for purposes of study and test in January 1942. The British strongly suggested that American ordnance officials confer with their representatives in order to accomplish an early standardization. It was further desired that all 20-mm aircraft cannon of this design procured for British use be of the Mark II type, the principal differences between the Mark II and the American M1 being pointed out as follows:
"1. The magazine carrier of the Mark II gun had a different latch; the ejector was provided with a buffer, and changes had been made in the magazine holding boss.
"2. A heavier rear buffer was provided in the Mark II gun and the back plate was dovetailed into the receiver instead of being fitted with a simple groove as in the M1 gun. Inertia blocks were used in the breechblock slides of the Mark II gun.
"3. The sear of the Mark II gun had been modified.
"4. Triple wire driving spring and extractor springs were used in the Mark II gun instead of simple single-strand coil springs which were used in the M1 gun.
"5. Minor changes had been made in the muzzle brake of the Mark II gun.
"6. The receiver of the Mark II gun was substantially different from the one used in the M1 gun; much heavier guide rails were used and the receiver itself was larger, heavier, and designed in accordance with British manufacturing methods.
"7. The chamber of the Mark II gun was 2 mm shallower than the chamber of the M1 gun."
The US basically failed/refused to make most of these modifications. Even with the AN-M3 (the US version of the UK's Mk V), many of these issues persisted.
The end result was that the UK never used US made Hispanos - even those that had been supplied to the UK under Lend Lease. Anthony Williams reports the Royal Air Force modified thousands of US built AN-M2s as ground AA (shortening the chamber and installing triple recoil springs), but none of these guns ever saw service.
I've also read accounts that US manufactured Hispano ammunition didn't play well with the UK built Hispanos. The RAF had a number of issues with US Hispano ammunition, to the point where they had to order inspections of the ammunition to eliminate duff rounds and then abandoned US ammunition for aerial use completely. The RAAF experienced something similar - their Beaufighters seemed to like US ammunition much less than UK/Commonwealth made ammunition.