I also have an ST90 with an engine that has no serial number. Probably used by the CIA for some long forgotten but strategically important mission. The bike and an agent, dropped behind enemy lines then had to escape across the border only to jump high barbed wire fences a la' Steve McQueen style.
That's the only plausible explanation that I can come up with but Darrel is more likely correct.
Rick
Two CT90 and two ST90 bikes