Here's a picture of one side of a MkII module:
I've circled differences in the duplicated image on the right-hand side. The orange circle shows where one leg of the IC has been cut and is left unconnected. On a MkI this is the leg which connects to one of the rear reeds, which is missing on the MkII(see next image on other side of board).
The green enamelled wire (circled in green) bridges the IC leg which normally connects to the left-away reeds on a MkI for hours adjustment, which is missing on a MKII(circled in pink), to the single remaining rear reed on a MkII.
The red circle shows the missing SMD resistors which are present on a MkI, paralleled with the 3 command reeds (integrated into the MKII chip).
The blue circle shows the missing SMD resistors which are present on a MkI, for light-sensor purposes (integrated into the MKII chip).
Here's a picture of the other side of the board:
The red circle shows the missing trimmer (the MkII is user adjustable) and the green circle shows the missing rear reed.
The MkIV is physically very similar to the MkIII, but as mentioned before has a square or encapsulated IC (not rectangular). Unfortunately I couldn't get any good images of a MkIV, but I've seen several PCBs used, including both the single sided and double sided PCBs (same as used for a MkIII).
Apologies for my crap photos. My camera skills are getting worse
Anyway, I hope this helps someone.
Rgds.