Miyata apparently started using serial numbers starting with letters in the early 70s. No idea why--they'd been making bikes for decades at that point, maybe that was when they started to export. Anyway the research of T-Mar, a C&Ver who doesn't post much anymore, revealed the 'code' as starting with A in 1971, IIRC. The following digits are the fortnight (2 week period) of the year, then the actual sequential number of the frame for the year. Bikes built late in a given year are usually for the following model year.
That's an old one, and looks to be in good shape!
__________________
I never think I have hit hard, unless it rebounds.
- Dr Samuel Johnson