FWIW: As I thought to have understood it, there are a few more differences for the the 3-digit POD:
- also uses another processor,
- added clip- & MIDI-activity-LEDs (the dots in the 7-segment digits),
- firmware update by MIDI/Monkey (i.s.o. EPROM-swapping),
- (so) does away with the internal backup-battery,
- specifies a minimum current capability of the 9V AC supply of 2000 mA (previously 1200 mA)
Apart from the current rating & clip- & MIDI-LED, indeed it doesn't necessarily need to be reflected in the manual .