Slotted rear brakes... Gotta be careful those bikes go so fast you have to watch for the brakes fadeing from heat... Awesome find. A single speed would be awesome. But how about replacing pads on it?
In the mid-70's JC Penny had some cheap ten-speeds with those rear brakes on them. They stop well if hooked up to decent levers. Mountain bike levers made for cantilever brakes seem to be the way to go with these.
I never did like the disc brakes when I ran pedicabs, only slightly better than cants when empty, but put 400 plus lbs of passenger weight in them and they heat and fade FAST. never had a fade problem with cants or calipers.