john
If you don't also have at least the carrier that fits to the back of the saddle as well as the part in the picture I'd advise against trying to reverse engineer the Boxford system and make new parts. Its very difficult to get right.
Because you have dovetail slides both below and above the central bar there is no vertical flexibility. So the system has to run absolutely flat and level in both sets of slides. The tail end bed clamp must also sit at exactly the right height. Tolerances are basically zilch!
My old SouthBend Heavy 10 had the same set up and getting it just right to get smooth movement with no drag both when ordinary turning and when taper turning was a right pain. Ant that was factory fitted kit exploiting SouthBends usual babbit filling technique to ensure the tail end bed clamp alignment was correct. Mine had been removed and refitted "not quite perfectly" by the previous owner to paint the machine so there was enough drag when in taper turning mode to twist the saddle a little upsetting the actual taper set and cut.
Not a great fan of having the loose tail end bed clamp dragging up and down the bed all the time during normal work either.
In your position I'd cannibalise it by fitting an angle bracket underneath to bolt to the bed and bolting either a U channel or square / rectangular block onto the taper setting part. Easy to allow a bit of space for vertical misalignment in a simple solid or U shoe follower. Its all lots easier. Thats the way folk like CVA, Holbrook, Pratt & Whiney (U channel) and Smart & Brown (rectangular block) do it. I figure they know what they were doing.
Never really understood why the system fitted to Boxfords became popular. Lots of extra fitting and precise work needed to get the sole, tiny, advantage of being able to immediately taper turn anywhere along the bed.
Clive