Sorry, I forgot to specify that I had in my mind Russian Airacobra when asking.
So, the Soviet manual should be a good clue, supported by the plausible hypothesis to avoid supply problems by retaining the 20 mm weapon.
I found a good article about ("
The P-39D-2 in the Soviet VVS"); unfortunately, it does not mention its armament. The only drawing there represents an M1 armed example, though.
Same opinion is of Massimo Tessitori on his "
P-39 Airacobras in VVS", where he interestingly details visual differences among early 37 mm delivered without protecting cover, later with, and long barrel of the 20 mm. Indeed, looks to me easy to confuse early 37 mm without cover
with "long barrelled" 20 mm.
although he rightly points out: "
It could be said in general that if "short simple barrel" is going through spinner it should be 37mm cannon and if "something long with complicated barrel" is going through spinner it should be 20mm cannon."
This doesn't mean that some, many or even all P-39D-2 couldn't be fitted with the Oldsmobile weapon, also by retrofitting; just trying to depict what I think to have understood.
Thanks gentlemen for your kind replies.
Cheers,
GB