It is called universal iris clamp or lens chuck.
They are available in different sizes.
The problem with lenses is that many of them are missing the lens flange. That makes it almost impossible to mount on a lens plate.
These clamps have very stiff aperture blades which makes it able to hold different sizes of lenses without using a lens flange. Most of the time these clamps are made of aluminium.
This beautiful clamp, made of brass, can hold lenses with a diameter from 2,5 cm up to 9,5 cm and up to a weight of aprox 2 kilo without any problem!
(BTW, this one is not for sale !)
I've never seen a brass iris mount before. Sweet.
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