Link to OP's photo gallery: https://www.bikeforums.net/g/user/518142

The serial number isn't quite was I was expecting. I'm not sure if it was misread or stamped incorrectly. While the component mix isn't correct for a 748, it does appear to be no lower a model and circa 1978-1979. What is the seat post diameter? Also, while it's almost certainly not completely OEM, what is the patent date on the rear derailleur and the date codes on the back of the hubs' locknuts?