Serial number states 1991 model year, but the model number looks older (as does the bike). These were very nicely made sports tourers...or fitness racers. Miyata made their own tubing (one of the few companies to do so) and the companie's background was actually in rifelry and artillery. A lot of us (me included) are fans of Miyata's bicycles as being tough and fun riders with a lot of comfort. They typically aren't light, but they ride well. I don't think RSX had STI in 1991, so I'm curious as to what the shifters are...some close ups would help. Shimano RSX was an entry level racing group below 105 that started around this period - 90' or 91'.

Is the first letter in the serial number a T?

This is bugging me...the color is correct for a 91', the internally routed TT looks right, the RSX bits look right...but the 91' catalog doesn't have the 712, it has the 714 , which is bonded aluminum.