I have a box of collets like these, but bigger, probably as described on your linked page. I always assumed they were the sort of collets you would use in a machine having a bar feed through the headstock. They can be arranged to close from behind so they don't retract, so the bar doesn't move back when you tighten the collet. The set I have came with an old Myford, which had no holder or anything else which could be used with them. Thanks to your link I can at least identify them. I've seen lots of these around but never seen them used.
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Looking around I found this link which includes the title of the standard - Collets - dead length type. This suggests that My earlier thoughts are along the right lines. Looking further I find which shows them being used on bar feed machines, and just about shows how they operate.
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This leads me to think abut the collets or whatever that are used on NC lathes, where I've seen them feeding the bar through while turning it to size, so the bar moves past the toolbit, which stays still. So the collet has to relax enough to let the bar slide, but maintain drive and concentricity. Something clever going on.
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Anyway, the DIN 6343 seems to cover a limited range of sizes. Maybe another standard covers smaller ones, or someone decided to make some similar to the standard ones. Then again I suspect quite a range of these collets were made before the standard existed, and I bet each manufacturer made theirs a bit different to everyone else, so you couldn't easily switch to a different supplier
