Leo Plas wrote:

It was I. The model number looks like it should be an L-0 mount, but
someone has removed the mounting threads from the od of the spindle end
of the chuck. Troublesome to fix.

Leo Plas

The model # is for a LO mount but considering the price in
1953 was $225 just for the chuck alone, rectifying the
thread deficiency would be well worth doing. A pinned
threaded ring retained by a circlip is not a biggie.

Tom