Zozo can say all about that.
What I have read from him is that ALL EP PCBs share the same circuit layout, with minor differences on the "fingers" of the EDGE connectors.
Starting from bottom, issue 3, 4 and 5 on the middle, and 6 the upper one.
The Issue 3 EDGE fingers were totally wrong, with excessive width, prone to cause short-circuits if not correctly aligned with the female connector . A bigger gap was added on issues 4 and 5 to fix that, and on issue 6 some of the unused pins of the Video connector were populated and gained a solder point, the planed use unknown.
There are also some differences on the custom chips mounted. Issue 3 were test EP64s fitted with Korean made Nick and Dave, first batches of chips that turned out to be malfunctioning, needing an external PCB to correct the behaviour. There are very few of these, almost all distributed to developers, not to open public.
Issues 4 and 5 mount the first batches of Austrian made custom chips, but it happened that the advanced designs were years ahead of the contemporary manufacturing techniques, and then the new Nick also didn't worked as intended. It showed bad colours on some intensive screen modes(and probably bad external colour input), some units worse than others. The cooper heat sink over Nick was then added to minimize the failure, worse on high temperature.
The new fixed Nick was introduced with the Issue 6 PCBs or a little earlier on last Issue 5.
There are other curiosities about the Enterprise PCBs, the most evident is the black lead from the Z80 to the cartridge port that can be seen on the upper side... the same cooper track already exist on the PCB, but cut on two places(on all EPs), a modification probably made to fix some insidious signal interference.