Yes it looks like a rather nicely homemade vertical slide. The T bolts would fit in the T slots on the cross slide, once the topslide was removed.
Whatever was used to hold teh work would bolt on the other round flange with the two curved bolt slots in it.
Possibly a three or four jaw chuck of some sort. Or a small vice with two hold-down bolts which could then be swivelled from a vertical to horizontal position. Or even a Myffrod dividing head looks like it could bolt on there, it used two hold down bolts and attached to the standard Myfrod vertical slide.
Is there anything in the box of bits you have that looks like it has two bolts or bolt holes that might line up with those two curved slots?