Definitely 1984 or 1985. Based on components, that should make it a 1984 model 710, as you originally suggested, but I would have expected the serial number to start with an M or N.
Also, is that blue paint inside the head tube? If so, that would point towards a 1985. For the 1985 model year, Miyata switched to a two letter, serial number prefix. It's possible they forgot to stamp the leading letter. The only wrong component for 1985 would be the crank (beside the levers, which are wrong for both years).