Dave,
It looks as if your slides have been modified so that the cable attachment is the same as used on Amal GP carbs. The 'hollow nut' is in fact a threaded nipple that should be soldered onto the end of the cable (in place of the normal 1/8 x1/8 sleeve nipple). This method eliminates the chance of the nipple getting dislodged from it's seat in the slide (as can happen if the inner cable has been bent at some time).
Regards,
Eddie
P.S. If you try to fit the 'hollow nut above a normal nipple, the nipple will probably land on the top of the choke block and prevent the slide from dropping fully.