Actually if you get a good look at the plane in person... it's just a blast tube sticking out of the nose, not a gun barrel. I'm willing to bet that if you pulled the cowling off, there's an empty space where the gun would be.
And as you can see in the photos the blast tube is of much larger diameter than the wing MG 151/20 barrel. A 30mm or 20mm shell could pass through the tube with equal ease.
Finally, about half of the D-12 had MK 108, half had MG 151/20.
The hole in the spinner is not sufficient to differentiate D-12 from D-13.
The only other spec differences I have found are in avionics and the engine, Jumo 213F vs. 213E. But even this was mixed - D-12s used both the E and F, and of the two D-13 prototypes, one was to use the E and the other the F.
I definitely think it is possible that JG 26 got one of the handful of D-12s or even a D-13 prototype. D-12 seems more likely because there is evidence that prototypes were completed and flown. D-13 prototypes appear to have never been completed, and series production certainly did not occur.
[This message has been edited by funked (edited 08-02-2000).]