Hello Anthony,
I had a Myford Swivelling Slide which I used on a Drummond lathe at the time.
I found that when using it, the slide had a tendancy to move, so I replaced the single fixing with two bolts. It seems that Myford have had the same idea.
My bolt had a seperate T nut screwed on the end, which I believe is slightly larger than a Myford one?
In our tradition of ‘never throw anything away’ I still have the original T bolt which I think is about 1/2″ in diameter.
Jim