Bill, I have 4 snapshots I have taken from the old Yamaha database site for the SA15 2 snaps and for the SA15D 2 snaps. One says that the model was released in 1968 and discontinued in the same year. The second one states the same but that the SA-15D continued in the Autumn of 1968.
The 3rd one say released in 1968 and discontinued in 1972. The 4th states released in 1968 and discontinued in February 1972.
My theory derived from my database is that all superaxes and the AE series were given sequential, unique serial numbers. If that is correct then this particular guitar is most likely a late 1971 based on the relatively high serial number.