Unimog Club UK

The difference in cabs is the 120 and the 121.

the 120 is the canvas top, although they're predominantly flat screens the doors are flat (no curve at the top), and they aren't as tall.
They also built some with a round screen (like your cab, which is a 121, but the 120 had a soft top), the round screen cab shared the same doors as yours.
 
The 121 is the hard top cab like you have, they did crew cab versions off this cab, including the 145 which was a 406 (short wheel base) and was only sold to the military).

so in conclusion, you can tell a 120 if it's a soft top