Supplemental Fan
 

My 1974 450SL (have had it for 1 month) does not have a supplemental cooling fan, is it supposed to have one?

Sure?
 

Sure you don't?
The second one is a small electrical one.
Open the hood and look from the frontside to the waterradiator, there you should see the small fan and motor. If you don't have it, somebody took it off.
But I never needed it yet: I heard it once for 20 seconds, after 30 minutes the engine on in Neutral with the AC by 35°C.
The coolingcapacity on the 450SL is much enough to do its job on his own.

Almost every 107's with 450, 350 and 280 engines never had electric fan. Also, the main cooling fan is permanently spinning.

Electric fan came with 380, 500, 560 and 420's together with temperature magneto-coupled main cooling fan (at least i belive it is magnetic coupling)