Here is your answer. Reading carefully, note the ROM tower or lack of it, and the upgrades in chips. That explains the differences in the A2000 rev 6 board visually. There are added components in these rev's, but the quick anser is TOWER and/or chips.
MOTHERBOARD REVISIONS:
Pre 4.x were German motherboards. Not recommended for use at all.
4.2
4.3
4.4
4.5 Last in the 4.x series
6.0 Has ROM tower for Kickstart, buggy with some expansion cards.
6.2 Removed ROM tower for Kickstart that was on 6.0 board.
6.3 Now has KS2.04 ROM, 8375 Agnus and 8373 SuperDenise as standard
6.5 Have not seen one
This came from a David Haynie file that I kept.
