Many Panzer III were armed with a short-barrelled 7.5cm KwK 37 L/24 gun. 700 were produced or re-equipped from 1942 and 1943.
However in late 1943 and early 1944 around 1/3rd of these were further converted with a lightweight barrel to a longer 7.5cm KwK 37 L/40.
The two tanks were then designated separately in a fashion similar to the Panzer IV F1 and F2. That being N1 for the L/24 variant and N2 for the L/40.
This used the same breach and ammo as the L/24 but allowed higher velocities and therefore higher penetrative performance for the PzGr. 39/43.
The barrels however were not made to last. But it was predicted that the Panzer III would not outlive the barrels lifespan.
This proved to be correct...
(Note this is fictional)
Also needs commander.