According to the reference source I have the Mustang Mk I had the W-7-B gun camera, the P-51-1 and the A-36A had the N-1 gun camera, the P-51A the N-4, the P-51 B/C the N-1, the P-51D/K the N-4 and N-6, and the P-51H the N-6.
I have what I believe to be an N-6 gun camera (I seem to have misplaced the P-51 that I must have bought with it, but I'll have to check my hangar at the airport again). I am am aware they made a similar type of gun camera but with the lens coming out a 90 deg angle as compared to the one I have; a friend of mine has one of those, I think.
It is likely that the differences in gun camera ports is related to the model of camera used, since there were at least 5 different models used.