Rubicon have indicated that they are doing the parts for an early M4A1 (vertical bogies, bolted transmission housing), so we could be looking at different M4A1 hulls - early small hatch with direct vision ports, later small hatch, perhaps with applique armour, and large hatch (76mm) version. If there is an early M4A1 in the works, I would hope it also includes the British pattern sandshields, although that might be asking for too much...