My first 240D was a '77. As I remember the knobs only operated the heater temp, not the ac. As long as R12 was easily available, this was a GREAT a/c system. The problem is that the York compressor just can't cut it with r134. Now that r12 is coming down in price, I see this system as desirable again.
A nice, low mileage early 240D with a stick and a good A/C that has never been converted would be a worthwhile car to have IMHO.
Have a great day,