I finally came across an eBay seller who was selling an original Steinberger USA leg rest. My 1985 XP bass now has the correct leg rest. Yay!
It came with aluminum inserts (not needed) and 7/16th Filister-head machine screws (pretty exotic and hard to find). Note, _machine_ screws, even though they are going into solid maple. They went in without too much difficulty, although a little flake of paint came off. But fortunately that spot is covered by the leg rest mount itself.
The main difference between these originals and the later ones used in the Spirits and Gibson-produced instruments seems to be the way the holes are drilled. The originals have a deeper carved-out hole that allows the entire head of the Filister screw to sit flush.
You can see an entry I wrote about the difficulties in finding and repairing these guitars here, The Steinberger Parts Dilemma. Here's a bit about the XP Bass. I am still hoping to one day get my hands on a matching XP guitar (not a Trans-Trem model, though; that's another parts nightmare I don't even want to contemplate!)