The 75mm turret was fitted to some large hatch M4A1s. They are sometimes confused with the M4 Composite hull, but there are several photos that clearly show the large hatch M4A1 hull. As Hoffman said, they were used as DD tanks; in fact nearly all the photos I've seen are the DD version. Here is what the Shadock site says:
The first M4A1s with large hatches were equipped with 75mm guns, and were manufactured by Pressed Steel Car in December 1943. They retained the "dry stowage" of the original design, where the 75mm ammunition bins were mounted in vulnerable positions on the sponsons. In the right side photo above, we've pointed out what we informally label "cast in appliqué." These "bumps" were incorporated into the hull casting in the area of the sponson mounted ammo bins, thus eliminating the need for welding on the one inch armor plates that were mandated to be factory installed or retrofitted on dry stowage Shermans starting in the Summer of 1943. It is thought that only Montreal Locomotive and Pressed Steel Car produced M4A1(75)s with "cast in appliqué" hulls. They appear on the last 75 or so Grizzlies (all small hatch), as well as the last 500 or so PSC built M4A1(75)s (approximately 400 small hatch units and 100 large hatch). Many of the large hatch M4A1(75)s were converted to Duplex Drive Swimming Tanks, and a couple of these DDs are on display in France and Great Britain.
