This is correct.
1957 this law was passed.
List of movies with switch blades in them:
1955
Bad Day At Black Rock (1955) Italian stiletto
Blackboard Jungle (1955) Italian-style stiletto
Dimentia (Daughter of Horror) (1955) Italian style stiletto
Kiss Me Deadly (1955) Italian style stiletto
THE Ladykillers (1955) kriss-blade stiletto
THE Night of the Hunter (1955) Italian-style stiletto
Oklahoma (1955) prop kaleidescope side-opener
Rebel Without a Cause (1955) Italian-style stilettos
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1956
Around The World In 80 Days (1956) Italian-style stiletto
Boy With a Knife (1956) Schrade? swinguard
While The City Sleeps 1956 unknown swinguard
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1957
12 Angry Men (1957) 2 identical Italian-style stilettos
THE Delicate Delinquent (1957) Italian-style stilettos
Gunfight at the OK Corral (1957) a bunch of Italian-style stilettos
THE Joker Is Wild (1957) American side-opener
THE Wild Town (1957)
I bet if you look up which year Mass outlawed suppressors, there would be a similar slate of movies with assasins quietly dispatching people with "silencers"