@Tankhunter__
I was being a bit sloppy with the term "prototype". I meant to just refer to experimental vehicles which were attempting to push the envelope.
Yes there were ~500 jumbos but they hard serious problems because the tank wasn't designed to move that much weight.
Yes there were ~1000 Panthers in France but they had the exact same problem
Yes there were hundreds of firefly (eventually), but the turret wasn't designed to hold a gun that big
The M4(76) was a design with all the kinks worked out so yeah, bad example in that case. But even though the 76 would eventually replace all the 75s, that upgrade process was just getting started in late 44 and it's not like a tank with a 75 would be at some massive disadvantage.