That twin spar YZ frame was a Japan only project from 1991-1992. From what I remember, the bike also had a...unique... head design that was somehow filled with nitrogen, giving the engine a variable compression ratio.
I was surprised to see how similar the 97-99 Hondas were in appearance. Too bad the Yamaha version was shelved.
The old YZMs were different animals. Water-cooled, power-valved, aluminum frame, VERY trick subframe/airbox combo, etc... Again, what could have been.