Hi Z shed
the Z1 tank looks the same as a Z1A or Z1B tank.
They all have the weldseam on the inside at the bottom.
The original Z1 tank has 122 mm between the two screwholes where the Kawasaki sign is attached , the Z1A and Z1B tank have 140 mm between the two screwholes.
I've seen some codes on the underside of some Kawasaki fuel tanks where the hook for the rubber band is attached but I have no idea what that code means.

That code is embossed in the metal and becomes visible when removing the paint to the bare metal.
GrtZ
Philippe