An interesting thread, and perhaps I could contribute a little to it
I have gone through my collection of Amstrad/Schneider CPC computers, and the number of revisions could be high. If You turn a CPC upside down, You'll find the serial number
, and on most machines also a MFR'S number or something like that. On many machines it is a yellow label with a letter.
My computers has the following numbers : Machine Serial Revision
Amstrad CPC464 535-8505506 B
Amstrad CPC464 148218 K31-4X ??
Amstrad CPC464 10631 K32-4X E
Amstrad CPC464 06730 -90-12 ??
Amstrad CPC464 24656 -91-01 I
Schneider CPC664 86340 K32-54 A
Amstrad CPC6128 133790 K32-57 A
Amstrad CPC6128 222758 K32-5X I
Amstrad CPC6128 222742 K32-57 I
Amstrad CPC6128 532 6805853 D
Amstrad CPC6128 542 9310181 K
Schneider CPC6128 136998 K31-56 E
Schneider CPC6128 151494 K32-57 E
Schneider CPC6128 532 6523111 B
464 plus 531-0782924 A (ESP)464 plus (board only : MC0122C 
6128 plus 531-0754346 B (FRA)
6128 plus 531-0760453 B (FRA)
GX4000 735-0919698 B
GX4000 735-0958752 C
GX4000 735-0959167 C
I have opened the GX4000's and the letters on the back does match the numbers on the PCB (at least for these machines)
I could make scans of the prints if anyone interested.