The Russians did put ShVAK into many of their Hurricane IIb's, after removing the .303 cal wing guns. Its certainly possible there were some field mods done, all I can say is that I have never seen a source for them modding the P-39s nose cannon. Even if they did, its unlikely it was a common enough practice that HTC is going to give it as an option. Your going to need a lot of sources to even begin to make the case.
The USAAF in the SW Pacific in 1942 did remove some of the 37mm in the P-39D's and replace them with the 20mm hispano as a field mod, but that option is already in AH.
I did find this:
"The only mention of this gun appeared in the book "Soviet X- Planes" by Gunston/Yefim Gordon.
According to that a Taubin 37 mm with 81 rounds was put into the Gudkov Gu-1 a design based in the P-39 airacobra. The emplacement was in the nose like the way the P-39 had its Colt M4 gun.
For several reason I think this is a mistake and probably Gordon mixed up with the 23 mm variant."
This was a planned Russian built version of the P-39, but they never built it.
...The Russians were big on field mods, and were not afraid to expirament, so I would hardly be surprised if they never tried to mount a few different guns on the P-39, but thats a far cry from saying "it was a standard mod" and finding enough proof to justify it in AH2.
