The single braze-on accepts a holder for compressed air cartridges. This feature was common on many of the mid and high range Miyata in the late 1980s and early 1990s. This was a nod to the triathlon crowd with which these models were popular. Unlike road racing, triathlon did not allow outside assistance for punctures, so carrying your own spare and means of filling it was necessary. The compressed air cartridges filled tires much more quickly than a pump, so they were a competitive advantage.

The 718A was built for for year only, 1999. But don't get the idea it was a failed concept. It got an extra cog and evolved into the 721A for 1990.

The concept of the 718A was to add a triple chainring to a lightweight, reponsive frame, thus permitting the avaerage cyclist to tackle big, long hills, without sacrificing high end performance. These bicycles are particularly well suited to rides like hilly centuries or for average strength cyclists who live in hilly country. At a claimed weight of 21.5 lbs, there is no arguing that the 718A is not a light bicycle.

The main triangle is aluminum which is bonded to a steel rear triangle. The presence of rack braze-ons on the seat stays indicates it was intended to carry a load, but I can't say how much load. I suspect it is somewhat less than a grand touring model and more along the lines of what you would carry during a century. Maybe a some rain gear, a change of clothes, your food and some basic tools. It would also make a great performance oriented commuter.

I doubt heavy loads would cause frame failure, but they may adversely affect the handling. Remember, this was still a light-weight performance oriented bicycle, so the steel in the stays is proably lighter gauge than what you would find in a grand touring model.

I trust this helps. Given that this bicycle has a known model year, it would be appreciated if you would post or PM me with the serial number for my database. TIA.