What I know about factory replacement engines is that they did not carry an engine number from the factory, this was left to the installing dealer to stamp in - this way they could insert the original engine number. So in effect this means that there could be two engines with the exact same engine number around, if the original one was later rebuilt. I have been picking up used engines for future rebuilding (I know, my hobby has gotten out of hand) and I now have three engines that don't have a number. Replacement blocks typically carry a little metal plate just below the flat machined surface where the engine number should be, that states that it is a "Tauschgerat" ("exchange engine" - sometimes this plate is in English).
There really is no difference at all between the basic sedan and the SL engines. For an SL, you need the tachometer drive (the sedan engines did not have the necessary gear set but this can be bought new and added, or just changed over from your old SL engine into the sedan engine). Then the camshaft may be different (can be swapped or bought new, although the 10 or so extra HP only come in at very high revs, where most of us never care to go) and the injection pump (just swap). Then finally the engine numbers are different, but that's really only cosmetics.
So I don't think there is any truth to the story that MB would prefer to put in sedan engines into SL's to reduce the likelihood of overheating: the engines are essentially identical. What is a fact, then, is that MB would supply the same replacement engine whether you had a sedan or a Pagoda: then it was up to the installing dealer to make the few modifications noted.
There may come a time when original engine number becomes important to collectors, however if you look at any advertisement for a W113, 'original numbers' is never mentioned. If it were to become important, it is apparently very easy to change an engine number (so I have been told!) so long as you have a grinder and a die punch set (any old hardware store). From the engine number, one should be able to tell the year it was built I suppose.
Sleeping sedan engines can still be found today in barns, garages and abandoned cars, at a reasonable price.
Cees Klumper in Amsterdam
'69 white 280 SL automatic