M-codes are codes for options that could be orderded.
The M stands for Mehr oder Minder , translated More or Less options
The list below tells you what the M-code means and the link will show you the parts that where involved in the package or option.

20 MPH Speedometer (export)
24 Sealed beam head lamps and red tail lights (USA & Canada)
36 Side flashing indicators for Italy
56 Steering/ignition lock
75 Mud flaps – front and rear
106 Heavy duty shock absorbers
119 Eberspacher BN4 gas heater
124 Yellow headlamp glasses
140 Fire engine as specified by German DIN specifications
142 Sliding door on both sides
151 Eberspacher BN4 gas heater
156 Larger oil bath air filter – 1500cc
161 Sliding door – Panelvan, 1-side
162 Sliding door on both sides – Panelvan
163 Sliding door – Microbus
164 Flashing front indicators, used from 06-04-1957
164 Sliding door – Deluxe
169 Sliding door – Kombi
172 Visible location of brake fluid reservoir (Benelux-Belgium, Holland and Luxembourg)
175 Overriders all models (Except types 26 & 265)
176 Same as M175 ( Pick-up and Double Cab, Export)
194 Heavy duty mirror arms
196 Asymmetric headlamps for RHD
200 Single cab with enlarged steel bed, Payload 940Kg
201 Single cab with enlarged steel bed, Payload 910Kg
203 Eberspacher BN4 gas heater
215 1 ton payload and 1500cc engine (until October, 1964)
216 1500cc engine (until October, 1964)
510 Upper divider between cab and cargo area
520 Sliding doors left and right
616 Back-up lights
620 12v system (standard from August, 1966)