www.w116.org/ should be a help.
1975 280 SE w116 engine number can be seen in between the engine block and distrubuter. It can be covered with dirt and it may be hard to see. To reach and clean it you should open the distributer cap.
I believe i may have found the answer in the user manual for the W116 at the following site: http://handbook.w116.org/Maintenance/MY72/120.pdf Good luck!
Latitude: N43° 36' (43.623187) Longitude: W116° 13' (-116.320325)