I believe that the Hawk81 reflects the version of the P-40B used by the AVG. There might be some minor differences in armor and armament compared to the actual USAAF P-40B, but I'm not sure. Maybe lack of self-sealing tanks?
Apparently, the really early versions of the the P-40 lacked armor and self-sealing fuel tanks, and just had 4 guns. They were used by the DAF under the name of Tomahawk Mk I.
Ah, this thread has the goods. The AVG Hawk-81s were ex-RAF with British .303 MG instead of US .30 or .50 caliber, plus an up-rated engine.
http://www.ww2aircraft.net/forum/avi...2-a-23934.html