Make your own but not from F/C steel.
BDMS will do and 304SS is even better for the deed. You won't strip bolt thread to easy.
I have original set from Beeston and they work well with topslide but another original set also from Beeston used with swiveling vertical slide was a miserable failure. Replacement also was a failure. I have found that these were made of F/C steel, rather bad idea for a part which is sometimes tightened with more heft.
I have centered tees carefully in 4 jaw, drilled out offending bolts, made similar bolts from M8 high grade stainless screw and brazed these together. Product seems to be undestructible. For topslide you will obviously use 1/4BSF or M6, not M8 like I did for vertical one.
Martin