A dab of solder in the end of the groove kept it closed in your pocket but made it easy to flick open. Some old-timers wedged a wooden match-stick in the groove for the same effect.
Now the only knife that is street legal in NYC is a four inch fixed blade like the Kabar...
http://www.bladeauthority.com/Ka_bar_TDI_Zytel_Handle_Plain_Plastic_Sheath_M_p/kb1480.htmd
That law is just plain silly. No matter how it opens, a knife is a knife is a knife.
"Ka-bar" means different things to different people. I seriously doubt that this Ka-Bar is "Street Legal" in NYC.