the Firefly was the British converson of standard 75mm gun tanks. They added the 17 pdr to 75mm gun turrets. They added these turrets to the following hulls: M4, M4 Hybrid and M4A4. The main differences in the hull was revised ammo storage. The 17pdr shells were longer than the 75mm shells. An additional bin was installed where the hull MG operator sat. This eliminated this position completely so the MG was removed and an armored plug was placed where the ball MG was. Besides this plug, there isn’t any obvious external differences to the hulls themselves.
There weren’t any put on M4A2s. Some very-late war (none seeing combat) conversions were done to American M4A3s but the info there is spotty.