For whatever reason, P-39N 'schemes go on the our Q model. 42-9399 is a P-39N-1 BE
To muddy the waters some more regarding P-39 serial numbers, the 43- prefix was never used. A large number of P-39Q's are in the 42- prefix range with 42-21250 being the final one before continuing with 44-2001.
So P-39N production began sometime in 1942 (there is a recorded incident on Dec 7, 1942 involving P-39N 42-4944) At some point in mid 1943, production switch to the P-39Q.
Here is an early production P-39Q with the simple U.S. roundel.
