Some good folks have taken it upon themselves to learn how to decipher Japanese serial numbers. They are usually semi-significant, i.e. the first few digits often represent the maker and the build period, the rest is just a serial number.
In this case the serial number is M8Jxxxxx: the M is for Miki, the 8 for a year ending in 8 (in this case I believe that would be 197
(edit: I was outsmarted by the editor. That was supposed to be 197
8 ) and the J for either the 10th fortnight (May 15-31) or the 10th month of that year. October is late enough in the year that they would have been producing frames for the 1979 model year.