I picked up a 15 or so year old Vision R-40 recumbent on craigslist, and can't find the serial number -- I've scoured the entire frame and don't see it. Any clues?

The only spot I've not checked is the chainstay has a protector on it (to protect from the chain) though that seems a really unlikely spot for it and I'd rather not peel it off unless I know it's there.