|While no one could have predicted the market for stratospherically priced old cars, stamps and other "collectibles" or "hedges against inflation" (more realistically described as, "neat toys for people with too much money and time on their hands"), I really, really doubt the emergence of a market in collectible C&C 27's. So, no, your boat isn't worth any more money but it may give you a useful conversational lever as long as you don't overdo it.
That is, of course, if there is any basis for the claim that it's a "first". A 1974 boat is not particularly old. Read the material in Overview and Evolution to get a perspective on 27 production.
If it is a 1974, your boat certainly would have the moulded-in HIN on the stern. It may also have a builder's plate in the cockpit with an embossed number. Either one might give you some data for assessing the previous owner's claim.
C&C 27 Mk IV