Your sample is a 1984 model.The Miyata 310 is lower mid-range model intended for club racers and avid cyclists. The frame is Miyata's proprietarey double butted CrMo tubing mated to hi-tensile forks and stays. Weight is about 25 lbs. Original price about $325 US.

The rear derailleur is a replacement for the original Shimano 105, as is the AX. The saddle is also a replacement.

Miyata are very well designed and manufactured bicycles, arguably the best mass volume brand of this period. It appears to be in very good condition, so provided it fits you properly and suits your needs, you should be very happy with it.