I believe that may be true. Most cars had troubles adjusting to the new emissions equipment required in the mid-70's, and mercedes was no exception. If it is the engine size that you like (for whatevere reason) you may want to look into the early 350SL's, as they also were 4.5 liters and are becoming sought after collector's items.
My other recommendation to anybody looking at that body style is the 560SL. This is the more modern of the cars, having all the power options available, heavier suspension, ABS, airbags, and a very powerful engine with a limited slip differential. Driving that car with the top down on a hilly, twisty road is like having your own roller-coaster! Yes, it does suck down a little more fuel, but all the US-spec cars of that bodystyle were very close in fuel consumption.
Check out
www.oursl.com for more details on the W107 Sl's. it is the best online resource I have found for SL's yet!