Pic assist
The bosses appear to be the same between front and rear. The stud and outboard location of the spring hole are unusual, but I'll bet someone here will recognize that brake design. First thing I'd do is remove one of the arms from the front, so you can better compare the bosses visually. You could even try mounting the brakes on the rear, just to see if they hit the rim properly.
I wonder if this is it -
https://www.ebay.com/itm/303504516991 . Still leaves you one spring short.