The serial numbers on both of my 1984 SBG1000 guitars are stamped into the back of the headstock, between the rows of tuners. My 198? RBX 350 bass has the s/n stamped on the back of the headstock. The serial numbers on both of my 1966 SG-2 guitars are stamped into the fretboard between the last fret and the end of the board. Same with my 196? SA-20.

In 1974, who knows? I would start looking at the back of the headstock, and then check the body end of the fretboard, between the last fret and the end of the neck. I don't think Yamaha has ever stamped the number on a detachable plate.