We use these for our own whelk pots and bait bags on the twine for closing up the nets, which is far easier to fit than the old rubber discs back in the Portzic days. Just squeeze these in the centre to fit the twine on and to slide release and they will grip the twine, if you want to grip them on firmer squeeze the ends in and it will lock the twine in tighter. We use these on 3, 4 and 5mm twine.
We only have these in boxes of 300 at the moment at £100 per box + vat . mainland carriage is about £6.50