Welcome!
I'll chime in here, if for no other reason, just to throw some other thoughts out there for consideration: I've
personally not heard of a cache of
any 1997 C5 convertibles that were factory-produced; and I can find nothing on the interweb supporting this guy's claim that any ever existed.
But, GM was known to produce pre-production runs of colors that were scheduled for introduction during the subsequent model year (e.g.,
the fourteen 1998 C5's painted in Navy Blue Metallic, which was a color being introduced in 1999); it's
conceivable they built some pre-production convertibles in preparation for the 1998 introduction of that body style. But like "6 Shooter", Bud ("Yoda"), and Jane Ann ("XLR8"), I'm skeptical... And I think you should be as well...
Obviously, anyone can visually verify whether the particular vehicle of which you're speaking is a coupe or convertible; Jane Ann's suggestion about acquiring the VIN is an excellent one; it can be easily decoded to provide the model year. Most likely, it's a '98 or later; if it
is indeed a '97, I would speculate that it's most likely a coupe that was custom-altered to be a convertible. Also in that case, more research would be required to verify the claim about a pre-production concept vehicle. Typically, someone making a claim about a vehicle like that (one of only 500 proof-of-concept pre-production vehicles) would have corroborating evidence (paperwork, etc.) to back up that claim. If he has none, I'd suggest to him that he produce some -- that can be independently verified -- if he expects to get a premium price for the vehicle...