A detailed answer would fill a book.
You’re basically correct on the distinction of the low bustle vs. high bustle. I’m not sure about the single-hatch being always low and the double hatch always high. I don’t recally ever seeing any photos to the contratry. Low bustle firefly’s carried two hatches, but I believe the Brits cut in the second hatch on those occasions.
Early/late depends on the context on what is being discussed.
There were three broad hull types: those that were a single casting, i.e rounded, which would be the M4A1, those welded from plates, i.e. angular which includes all models aside from M4A1 and late M4, composite hulls which had a cast front welded to a welded rear section which are found on the late M4.
Early hulls had a steeper slope to the front armor, 56 degrees vs 47 degrees, and had small forward hatches that hinged on the side, while the late hulls had larger hatches that hinge on a a corner. The extremely early hulls had direct vision ports in front of the driver’s and gunner’s hatches while the later early hulls had a fixed forward periscope instead.
As far as I know, all wet storage hulls, in which the ammo is stored in water jacket bins below the turret are late model, whether cast or welded. The dry storage, in which the ammo is stored in bins in the sponsons as well as on the perimeter of the turrt basket would include all early hulls, the composite (which only came as late verison), and the uparmored Jumbo Sherman.
Early transmissions were prominently bolted together from three castings, intermediate transmissions were made from a single casting but were still quite rounded, whereas late transmissions were made from a single casting that angled to a fairly sharp point.
You have two basic categories of suspension bogies: VVSS (vertical springs) which had narrower tracks and in which the tracks’ guide horns were integral with the end connectors and went outside the wheels. Later on, HVSS (horizontal springs) was introduced which gave wider tracks and in which the guide horns were a single row, running down the center of the track.
The VVSS was most common and had many variations. Early suspension bogies had the return roller mounted above the center of the bogie and were found only on the early M4A1. Later bogies, put the return roller on an arm that trailed back, and also included a skid over the center of the bogie. The earliest of these later types had a small round skid and a return roller are that went back level. Later on on the skid becam larger, and flattened on top. The roller was then raised from the arm with blocks, and finally the arm itself was sweptback at a rising angle.
There were several types of engines as well: a 400 hp radial aircraft engine for the M4 and M4A1, a pair of diesel engines for the M4A2, a Ford GAA V8 for the M4A3, and a multi-bank Chrysler (made from 5 engines wrapped about a common crankshaft) for the M4A5.
There were a few guns as well. The most common being the 75mm, a high powered 76mm found on late hull M4A1s, and late hull M4A3s, a 105 howitzer. The British cobbled in a high powered 17pdr to make the Sheman Firefly.
Some of these various options are mutually exclusive, but for a lot of the others it’s mix and match, both form the factory and in the field. For example,
You could have an early M4A1 with direct vision ports, bolted trans, and early bogies. You could have an early M4A1 with fixed periscopes, bolted trans, and early bogies.
You could have an early M4A1 with fixed periscopes, early cast trans, and early bogies.
You could have an early M4A1 with fixed periscopes, early cast trans, and somewhat later bogies.
You could have an early M4A1 with fixed periscopes, late cast trans, and late bogies.
You could have a late M4A1, late cast trans, and late bogies.
etc., etc., etc.
The possibilities are endless.
Andy