Your best bet is to contact Yamaha and see if they can identify it. I have a Yamaha FG, having trouble with the truss rod,I cannot straiten the neck.
Inside there is a brown label with a black scroll Design motif and GA written in gold, the serial number inside is. The latest is an FG with no date code. The second two numbers are the month; here, May. The first number is the year; I found a Japanese guy on Youtube who made videos of himself playing many early FG, and he mentioned the serial number and date code most of the time. The first 2 letters represent the year following the same coding system. The last three numbers are the unit number.