I think it is called a window sash vent stop

A quick learner from New York City says:
I always called it a flip lock, so you could safely leave your window cracked open at night on when you are out. It has a number on it in a circle C-86 with a 5 underneath
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Paul from SWISCO responded:
I'm not sure what you have, exactly. Could you send us a few pictures of the hardware in question? Thank you!
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