Needed a small four jaw for the Summit, the 16 inch will only close to a little over an inch, padding the jaws or chucking a small chuck was getting old. Found an 8 inch Chinese one for $114, figured it can't be too bad, can always grind the jaws if they aren't parallel.
Had a D1-8 backplate I believe from a chuck of some sort. A little rusty but Evaporust and some polishing on the the mounting surface and taper cleaned it up nice.
There was a centre boss on the front, the outside face was hardened for 50 thou or so, nice glowing swarf till you broke through.
Finished facing it and managed a small boss to locate the chuck of its inside bore. The back is otherwise flat.
Once I get them bolted together I'll hold the chuck body in the big four jaw and cut the OD down some. Can't quite get to the dia of the chuck, the pins are about 7 3/4 on the outside, 8 inch wouldn't leave much meat. Couldn't turn the OD when I had it mounted, the spindle flange is about the dia of those outside bolt holes.
Greg