If I recall correctly from a conversation I had with Gene, the quick answer is that the top photo is what was referred to as a "shrouded, two-piece" design w/ the McCkicky switch. The bottom photo is simply the shroudless "one-piece" version w/ McClicky that was in place before and after the shrouded version.
I don't remember the particulars, but I do know that the two-piece tailcap has a removable shroud surrounding the switch boot and could be unscrewed to easily switch out the boot. However I seem to recall that producing this two-piece design was more of a hassle than it was worth (my words), so production of these was limited. So back comes the one-piece w/ McClicky and based on my samples of these, I don't see any other differences.
Prior to introduction of the McClicky switch, the Judco (Kroll) switch was used in the first iteration of the MD2 body that did not have a separate tailcap. But a separate tailcap with the Judco (Kroll) was used on the MD3 & MD4. The separate Judco (Kroll) tailcap was then used on a new MD2 battery tube, thus bringing about a two-piece MD2 sans the head.
If more clarification is required, Gene will probably check in here with the answers.
-John