The collet on the one I had many moons ago had a fine thread on the rear end of the collet and screwed into the taper and a hex on the front to tighten it up. It took a 1/4" shank.
I must say out of the 10 years I used the Deckel I never used that attachment once.
There a couple of pic here to give you an idea. http://veltmanmachines.nl/en/vm/milling-machines-accessories/hoekfreeskop-deckel-eckenfraskopf-2036/