Hey Tom, am I to understand that ANY locking blade is illegal in NYC? That's what your link says, but it strains credulity. Regarding "possession," as I wrote, even if you keep something illegal in your home, you needn't worry so long as a LEO doesn't visit you. As for butterfly knives (which, for the record, I think are silly), your link had this to say:
Butterfly or "Balisong" knife, a folding knife with a split
handle, did not have blade released by force of gravity or
application of centrifugal force, and did not constitute a
"gravity knife"... (1987).
Thanks for the handy link.