From the context Bob, I assume he means a chuck. (It looks to me like a translation from German). The original Unimat SL chuck is fixed to its M12x1 threaded backplate by three screws. When used on the indexing head, the backplate is removed and the chuck is bolted to the indexing plate using the same three screws in the holes visible in photo 1.
Brian
Correction: I think the index head comes with three longer screws used instead of the backplate screws.
Edited By Brian G on 29/07/2018 19:54:22