I believe SE/A is a normal head designation and does not mean your head was swapped to a non-SL one.
I think your mechanic may be asking about the timing of the camshaft, vis-a-vis the craknshaft. There must be markings on the camshaft wheel and crank etc - but the mechanic can also just make some temporary marks before he removes the timing chain. I don't know off the top of my head what/where the timing marks are, but the various shop manuals (Haynes, Mercedes, Chilton's etc) have those instructions. Maybe someone here who knows better, or can provide a link to a forum thread, about the engine timing.