In the early 70's I went to Scott Community College in eastern Iowa. They had a motorcycle mechanics program that I was enrolled in. They had a Honda 750 that we were not allowed to mess with. I do not remember the color, but I do remember the oil tank had an aluminum cap. I think I remember that it had a drum front brake, maybe. By the 90's, motorcycle program was long gone, and the people at the college likely did not know what it was.
NOTE: I am not saying it was a sandcast prototype, because I was not that well informed at the time.
I do recall it was different than regular 750's, and there was talk that it was given to the school by Honda.
A rule for the gift was that Honda said it was never to be disassembled.
Steve