the e2 and zero-b boards are near identical. the only real difference is one says E2 on start up and one says Zero-b.
there is no "e1 zero-b".
theres E1, E2/Zero-b, and tadao's Yakuza. and any of the boards will drop right into any of the eblade frames. although the screens on the e2/zero-b/yakuza are bigger than the e1, and the E2 frames and some worrblades have a bigger window for them. even if your frame has an e1 window on it, you can still see 95% of an e2's screen.
E1 boards get terrible battery life. like maybe 2 cases per 9v. everything else gets like 15 cases.
left is an E1, right is an E2:
notice the e1's chunky trigger, and the trigger guards are different. e2's also have a teardrop shaped cutout on the top button.
this is a worrblade, wgp's take on an e1 frame:
has a square trigger guard, and a built in rail on the bottom. takes e1 triggers. came stock on the karnivor, later models came with e2's in them and had the bigger window too.