From Wikipedia: "M4A1, M4A2, and M4A3 models received the larger T23 turret with a high-velocity 76 mm M1 gun, which reduced the number of HE and smoke rounds carried and increased the number of anti-tank rounds...The first standard-production 76 mm gun Sherman was an M4A1, accepted in January 1944." The muzzle brake was added to redirect the muzzle blast to the sides after it was found that without it no followup shot could be taken due to dust and debris obscuring the target. In all about 4,500 of these tanks were built, but I could not find how many found their way into theater. However, after the heavy tank losses of the Battle of the Bulge, in January 1945, General Eisenhower asked that no more 75 mm M4s be sent to Europe: only 76 mm M4s were wanted. The Jumbo was a very rare variant with upgraded front glacis armor built to assault heavily defended areas.