Looks like an original stamping to me as well! That frame is nothing like a standard 500 though. Over here i could write to Honda UK and they will email Japan--for a price of course-- perhaps Honda USA would do the same? The other possibility is it is a Yoshimura supplied engine and maybe he had "proper" stamps and used new casings, dont know as i only ever bought odd parts for customers who never wanted it all and whinged when what was fitted didnt work properly! There was a "tuner" in the new york area, Mark Mcgrew i think, who reputedly worked as a tech rep for Honda US back then perhaps he can help, unfortunately i no longer have his email address due to several Bill Gates inspired failures!!!