Welcome to the Forum !
MOD veh's are either G.S. (General Service) 12v or F.F.R. (Fitted for Radio) 24v.
The wiring and peripherals (starter/wiper motor/ignition system) is the biggest difference. Most of the 109's have heavy duty suspension, extra thick leaves, longer shackles, longer spring mounts on the chassis.
The MOD's mostly have a 6 position light switch, instead of the civvy rocker/lever switch and thus the wiring is somewhat unique.
Body wise the panel work/seats are inter changeable.
Hope this clears the water, not muddies it !
Les Parker
Tech. Support and Parts Specialist
Rovers North Inc.