What is this SINAR piece?

[SALTZMAN]

Hello.

I inherited a bunch of camera/darkroom equipment from my late uncle, and have no idea what most of it is.

Can someone please tell me what this contraption is called? The diaphragm-y thing is marked Sinar...I'm not sure if it's removable from the hinged piece or whether it's all one. There is a small piece of white label on which my uncle labeled "58mm."

Thank you very much for your help.

DJ