If you're talking alto and soprano clarinets, I can't think of a reason why this'd be the case. It's true that Vito bass/alto clarinets do occasionally have some odd (patented) keywork. That might be it.
In this case, I don't know how old this particular alto is, so I have no idea as to what the value is. Just based on seeing how sharp the Selmer stamp is in the upper joint would lead me to believe that it's not very old -- but you can always get a gold crayon and renew the stamp.
Terry does point out an awful lot of alto clarinet auctions, tho.