HELP NEEDED Identifying Brake Spindle - likely from a late 70s Volare

So, what brake calipers would fit a FMJ spindle???
We've tried the supposed single piston replacements, and they don't seem to fit correctly.
(I had ground those off to clear the wheel edges - had also already bought 14 inch tires!!!)

Anybody know anything about an "Optional Mounting Adapter"????