I also have a Super Le Mans with the serial number on the seat tube. I got it while dumpster diving, so I only got the frame and stem. But it looks identical to rhenning's. The serial number is N645. The stem is an SR Forged stem with the date code 75.4, so that would make the bike a '75 or '76.
Also the paint and decals are in bad shape, and I would like to do a repaint with new decals, but a search for Centurion catalogs turned up nothing. T-mar, do you by any chance have a catalog from these years.