So I purchased this mini-art Zis-3 and I had not realized it, but it looks like it's the parts for either a zis-2 or a zis-3. I gather from comparing instructions that the shield and gun are different but that the carriage is identifical. I may have not read or understood them correctly. I don't have any separate resources on these so have no way to check. So, were the carriages identical and is it just a matter of selecting the gun and shield combination to go on them?

I imagine it's not going to be quite as simple as that of course....

Wikipedia says:
"The ZiS-3 combined the light carriage from the 57 mm ZiS-2 anti-tank gun and the powerful 76.2 mm barrel from the F-22USV, the previous divisional field gun. The addition of a muzzle brake reduced recoil and prevented damage to the light carriage upon firing. Producing a ZiS-3 cost only a third of the time and two-thirds of the money of a F-22USV by making greater use of casting, stamping and welding."

https://en.wikipedia.org/wiki/76_mm_divisional_gun_M1942_(ZiS-3)

To strip that down:

ZiS-2 - 57mm gun
ZiS-3 - 76.2mm gun

KL