So which M4(mouthful) do you
actually want then, what start date, and as I would not know a "cast hull" from a "welded hull" if it bit me, what armour scheme?. Is it just a copy and paste of an existing M4(mumble) with a new gun and name? - if so, just say "Copy unit #xx, give it the L40 gun and the jumble of letters and digits becomes M4Abc123"...
(I can see why we Brits renamed US things with real names in WW2, not some M-number and a jumble of letters and digits..
Which M3, did sir mean - the light tank M3, half track M3, the medium tank M3, the M3 SMG, the Tractor Crane (2 ton) M3. Much easier to use proper names for things!
)