HelloGeorge,
The 280 SEL/SL is the head designation and is cast on the head as in the photo. Refer to the block number below, since the engines on all the SL, SEs, and SELs shared the same heads. There are different versions (early and late) but these were used on all models depending on the year. The stamped number on the block deisignates the actual "engine number" which will indicate the model application for that particular engine. The engine number will tell you what model the engiine came out of.
I would be interested in getting the ten digit part number cast into your head for my "cylinder head chart" for comparison, since this is most likey an unusual variation.
The cast iron 300SEL engine is quite unusual and you may have trouble getting correct information on it. It was used in very few of the latest 300SEL sedans. It is basically a M130 (280 series) cast iron engine used to replace the extremely expensive aluminum alloy block engine used previously in these sedans.