IMO, IANAL, possession means on your persons.
But welcome to NJ laws where everything is gray. If you ask a cop, a lawyer, or one of us, you will probaly get 3 different answers.
My opinion "prohibited weapon" would be a secondary charge if you commited a crime, and they wanted to pile on charges.
When you say push button dagger, you mean a switchblade? I wouldn't recommend carrying it with you daily, but if you bought one and put it in a case on display, I doubt you would get in trouble. Unless your house gets raided often.
Again I am not a lawyer, if you feel there is a risk of owning something, don't take that risk.