I have the same guitar--it was my mother's many years ago--she got it as a high school graduation gift.

I play it primarily now, and it's a decent guitar for what it's made of. All laminate, no truss rod, but has a warm, round sound. It's also a little bigger than traditional classical models.

If you look at the serial number, you will be able to see when it was made; the first digit is the year it was made in (this guitar was only made from 1970-1975 as I understand), then the next 2-4 digits indicate month and day, and the remaining digits indicate number.

For example, my serial number is 0551690
0 = 1970
5 = May
5 = 5th
1690 = number of guitar produced.

I really like it and though it's forty years old now, the neck hasn't warped. The action is a little high, but I don't feel like filing down the bridge at this point... regardless, it's a great guitar to learn on, but probably isn't worth more than a couple hundred dollars.
I have a Yamaha G-90 as well, but my serial number is 60826140 - how does that tell me the year it was made? It doesn't seem to follow your pattern.