No, It was actually the second main hull design, though still early in the war. It started with the M3 Lee medium tank, which was first designed with mostly riveted armor. Later, they designed a welded hull version and also a cast hull version.
The idea of the cast (rounded) hull versions, both early and late, was to reduce shot traps (areas where an incoming round would be less likely to deflect due to a obstruction), mostly on the front of the hull. By the time the M4 Sherman came around, riveted armor was obsolete. Most shermans were welded hulls, since the only manufacturer able to make cast hulled shermans was Lima locomotive in Lima, Ohio. Both welded and cast hulled shermans were produced at the same time during the war.
Later in the war, both hull versions (cast and welded) were redesigned to have a slightly less sloped front glacis plate to allow wider driver and assistant driver hatches, eliminating the old and stupidly desinged direct vision boxes and their aplique armor plates. That alone saved many lives, since the early hatches were very small and hard to get out of...especially when injured.
Most shermans with the late hulls were upgunned with the 76mm main gun and the newly designed turret to accomodate the new bigger gun. Also, most had the new HVSS suspension with the much wider tracks.
The variants break down like this...
M4-------Welded hull version with 9 cylinder radial engine.
M4A1-----Cast hull version with 9 cylinder radial engine.
M4A1E8---Redesigned and upgunned cast hull with 9 cylinder radial engine.
M4A2-----Welded hull version with twin 6 cylinder GM diesels. GM manufactured.
M4A3-----Welded hull version with Ford GAA V8. Ford manufactured. (Most common, and most reliable.)
M4A3E8---Redesigned and upgunned welded hull with Ford GAA V8. Ford Manufactured. (Also most common and reliable...and the final version used by the US)
M4A4-----Welded hull version with chrysler multibank engine (five inline 6 truck engines bolted together). Chrysler manufactured.
M4A6-----Composite hull (front cast, rear welded) with caterpillar diesel. Very few made, and extremely rare. Basically a experimental model.
The M4A3E8 was the final version of the sherman made, and at that point it was refined and was as perfect as the sherman could be at the time. It served through Korea and was replaced by the M47 and m48 Patton series tanks.
Just about all the shermans we used in Korea were the M4A3E8 versions, probably with a few M4A1E8 mixed in there. I don't believe there were any 75mm versions used since they were just about obsolete at that point. The 75mm was a low velocity short barreled gun that was already almost obsolete towards the end of WW2.
The one in the picture you posted is a Late war M4A1E8 cast hull. It's got the HVSS with wide tracks. The redesigned hull with wider hatches. The new, bigger, 76mm gun in a newly designed turret. Powered by the 9 cylinder radial engine. It's possible that it did time in Korea.
Hopefully this is somewhat easy to understand. I know I ramble on about this stuff like people know what I'm talking about....
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