I believe the original thread plates for the strikers were 1/4-20 (I can go confirm that if necessary.) But in either case, it wouldn't be difficult to fabricate replacements out of some bar stock. They have a serrated front face to keep them in place as the strickers are tightened, which might a little tough to replicate. Maybe use a chisel to upset some metal that would provide some "bite" into the sheet metal of the pillar. I think it would be a giant PITA to use nuts behind the strikers, as the striker would cover up the access to reach the nuts.
