Originally Posted by FBinNY

These were common items in their day. Coaster brake bikes often had a vertical hole in the brake bridge to directly mount fenders without a bracket. To fit these the brake's pivot bolt was fixed into a alloy block that wrapped half around the top of the bridge, and was drilled for the vertical mounting bolt, or on some versions had a vertical stud integrated into the block.

And yes they're probably still around, at the bottom of the scavenged parts bin of any shop that's been around for 30 or more years. I might even still have a few in my Dia Compe small parts drawer, & I'll bet that Peter at Cane Creek (formerly DiaCompe USA) could put his hands on one on a minutes notice.