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Was your father a WWII veteran in the European theater?
I do not know the current state re' the legality of gravity knives. They were illegal. However, at a recent gun show, I saw multiple tables of spring-loaded, out-the-front switchblade knives offered with no caveat whatsoever.
I had one of these, given to me by an elderly man at church. He had taken it off a German P.O.W.
It was not the take-down variety.
I waited until Mr. Cook had passed away and I sold the knife to a knife dealer.