Originally Posted by Pruitt
If you look further you might find that the same cannon is better known as the French 1897, made famous in World War I. The 75 came in a M2 and 3 which went into the M3 series tank and M4 series tanks. The gun was also lightened and modified to be fitted into B-25's and the M-24 tank. The gun could penetrate Mark III's and Mark IV's up to the G model.
The M6 was better known as a Infantry Support weapon. Newer 75 and 76 guns were better at armor piercing.
It might be a good idea to remember that the majority of the use of guns on tanks was not in anti-tank use. The HE use of the 75 was what made it so effective.