Ralph.
see link, it?s a typical Stevenson Block you can purchase from Arc Euro.UK and many other tool houses.
Note the overhang, also if holding the blocks horizontal in standard vise, the nut is larger in diameter than the flats
this can be an issue and you then have severe overhang, and dangerous, IMHO this standard ER external ring style is suitable if the block is standing vertical only.
The internal locking ring type i much prefer.for horizontal use.
It?s all a matter of fit for use.
https://www.arceurotrade.co.uk/Catalogue/Collets/ER-Collet-Fixtures/Stevensons-ER25-Collet-Blocks
Thanks
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John
John Lindo