The mechanical odometer used in all Mercedes is a direct descendant of one sold by VDO to Porsche and Mercedes since at least the early fifties. Somwhere in the 1975-1990 timeframe they changed the drive geat from zinc (which wore out) to some sort of plastic (which wears out). If your odometer doesn't work but your speedometer does this is probably why. I don't know if you can get just this gear - which is all you need and you may need to find the elusive "dashboard overhaul kit" or just get a new odometer complete assembly. Here's Peter Fredricks article about the earlier zinc ones.
The drive gear for the odometer is slipping on the shaft, so the wheels don't turn to change the numbers.
The easy fix is a small amount (less than a drop) of thread sealant (Locktite or equivalent) on the interface between the odometer shaft and the zinc drive gear.
You must remove the speedometer to fix this, quite a pain, but if you've done the heater, you can do this.
Remove the parking brake bracket and shove you hand up to the back of the instrument cluster and unscrew the white plastic knob there. This holds the cluster onto the dash. Reach over and unscrew the speedometer cable from the back of the speedo head.
Pull cluster out onto the steering shaft (you may want to pad it to prevent scratches) and remove the wires and the three cross-head screws that hold the speedo in and gently remove it. The knob for the odometer reset stays on the face. Have a care not to pull the chrome trim ring off the front, the glue is brittle.
Once you get the speedo out, you will see the zinc gear on the passenger side of the numbers (beside the 1 mile wheel). This will spin freely on the shaft, allowing you to change the numbers.
Using a toothpick or tiny brush, sliver of wood, etc, apply a very small amount of LockTite or other anaerobic thread sealant to the point where the gear and shaft meet. Work the shaft back and forth a bit to get the sealant between the gear and shaft. Carefully wipe off any around the point where the shaft comes through the bracket.
When the sealant sets up, you will no longer be able to spin the gear freely, and if you carefully spin the speedo (positive direction!), the odometer will again advance.
Re-install and you are set to go.
Takes about 5 min once you have the speedo out......
1972 280 SE 4.5