Bill
I don't know if this will be any help but I have a 1971 280SE 3.5 with dealer fitted air conditioning that sounds like the one you describe. It is a Smiths Model 649 underdash unit. It is about 45 cms across and has two round controls (fan and temperature) on the right hand side. I have the original manual and warranty cards as well. It looks like models 219 and 429 would also fit. These units were likely to have been around when your 280 SL was new. If you really want to go down the path of retro fitting air to your car, then you'll probably find a complete unit at a Mercedes wrecker as air conditioning was a commonly dealer fitted accessory to the sedans of that era.
Be aware that the early piston type compressors take a lot of power and you might want to update it to a rotary type.
My 1965 230 SL also has retro fitted air but this is a modern Sanden system installed by a previous owner about five years ago.
Regards
Mark
1965 230 SL light blue, auto, RHD
