Does any know what these are called?
They are alternatives to ER collets but stiffer, and usually with better accuracy.
They fit into any suitably-sized standard ER socket, but instead of the30 degree bit at the top they have flat tops. The nuts are different to standard ER nuts.
The main disadvantage is they don't fit a range of sizes like ER collets, instead you need a collet for the exact size of tool in use.
They work because ER collets can twist a bit, and these can't.
ER Collet, smaller size:
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Note that the top part is roughly ball-shaped, and especially when the tool isn't inserted the full length of the collet, or when the hole in the collet isn't all full-length, ie it has a wide bit at the bottom as in the diagram, then the collet can twist sideways a bit.
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and the nut fits flush on the flat top, preventing the collet from twisting from side to side.
-- Peter Fairbrother