Paging through my copy of "Journeys Under Difficult Conditions" (August
1984 edition) I came across the following M-Codes:

M-032       Lockable Fuel Tank Cap
M-060       Auxiliary Heater
M-103       Stronger Coil Springs (Vanagon?) &  HD adjustable Koni Shock
absorbers
M-121       Additional Fresh Air Fan (if not already standard)
M-240       Recessed Crown Pistons (all carburator engines except
Volkswagen
                  Transporter 2.0 litre)
M-551       H-4 Halogen Headlights
M-561       Hinged Side Windows (if available)
M-616       Back-up Lights
M-622       Socket on Instrument Panel

I know that someone (Clara?) is keeping a list.  Hope that this helps.

Jim Arnott
Union, OR
'68 Type 1
'77 Riviera II
'83 Vanagon L (2)