For sale is a Bridgeport Mill X Y Ball Screws. These fit a 9x42 Bridgeport, as I ascertained by laying the X screw along my own mill's table, it was exact size as my screw.
I bought these screws at an auction conducted by Triton College, they surplused a big part of their machine shop. According to the people there, these screws are new. They surely look like new to me as well and operate smoothly. There are very minor specks of rust on them, but nothing major.
These screws could be useful in renovating your mill, or in converting it to CNC.