I also have one of these very good Burnerd 4 jaw chucks which are directly threaded for the Myford spindle nose: also a Burnerd "Griptrue" 3 jaw that is similarly threaded without a subsidiary backplate. When I bought them new, in the mid 1970's, the 4 jaw was £18.50 and the Griptrue was £31 . The sheer quantity of Myford lathes in use must have made it a proposition to produce the internally threaded bodies rather than the more usual backplate only chucks. Dave