I recently bought a used 250mm 3-jaw prattish chuck and backplate for the backplate so that I could mount an old 4 jaw chuck to it and keep the chuck as a spare.
Unlike the "proper" chuck on my Triumph 2000 (which has a separate backplate), the backplate is integral with the rear half of the chuck. Although I could machine off the shielding parts it seems a pity to destroy a fairly reasonable chuck.
I'd be happy to swap it for a D1-6 backplate if anyone is interested.
Bob