Never heard of Bridgestone? Perhaps you've heard of the tires they sell in the US under their Firestone name.

Bridgestone also made fine bikes. Mountain Bikes (MB-x), City Bikes (CB-x), Road Bikes (RB-x), and Cross Over (XO-x). The lower the number the higher quality the bike, though in general the top end shared the same frame and differed only in components. As far as I know, the zips were after-the-facts, tacked in as a better-than-the-best kind of offering. I know the MB-0 came along long after the MB-1 through MB-5 were established. I don't know if there any others.

Good bike, if it's in decent condition and cheap, it may be worth not converting.