The production M51 was based on late model M4A1 'big hatch' examples that the IDF obtained from French surplus in the late 50's / early '60's. As such I'd contact Forgebear and see if he still has some Mato M4A1 upper hulls. Failing that Nick makes some very nice resin casts, but they are considerably more expensive. As I said, Frankie here on this board is the master of making small hatch M4A1's from the big hatch hulls - so long as you don't want to get to picky about the angle

If you want a 'correct' small hatch cast hull, then you are basically out of luck at this point in time
The bar cleat tracks in your photo are also out of production to the best of my knowladge, and in short supply -, again if you are in the UK/EU, check with Forgebear as he may still have some knocking about?
Sorry I don't have better news, but small hatch, early turret/mantlet Shermans are off the beaten track right now
