So I guess you bought it from Roy, the guy who sent me the pics?
I can't tell whether it is true it was a prototype. Three things:
- The serial# starts with UB, what is slightly earlier than all my '93 frames (UN). This makes (late) '92 plausible.
- I have never seen that size Elev8k with that short headtube. The 16.5" came with with a headtube that short, but that deffo isn't a 16.5".
- Miyata also supplied brands like Koga and Univega. If the bike was on display on an exhibition, it could have been a B to B exhibition. I can image a company will display it bare or with more informative decals than.
Decals: gil m from here does decals, but I doubt Miyata decals. Another possible source => Velographics
Enjoy it! Your bike shows it looks ace also with modern stuff on it.