That's a Shimano disc brake... I used to own an NOS Shimano OilLite full hydraulic rear disc brake, the caliper was similar to that cable version and the rotor was identical. I traded it to Thorsten for a Magura Clara disc brake.
10-12 years ago we bought close to 300 of those old Shimano disc brakes still in the original master cartons of 50. We've built them into all kinds of wheels over the years. They actually work surprising well with a good quality brake lever and lined cable housing.
1973 Schwinn Lemon Peeler with rear disc brake (available on '72 and '73 Krate series bikes), picture scanned from Liz Fried's excellent 1997 book entitled simply "Schwinn Stingray". Photo by Jeff Hackett.
The rarity of those "Krate" frames is due in part to the fact that they had that drum brake on the front. In the '70s you could get the bikes cheap at a garage sale, throw away everything but the front hub, drill eight more holes and build your klunker front wheel.
I don't know that I ever saw the disc brake version. Was that a limited production?
The reason the Krate bikes went out of production was because the CPSC didn't care much for the shifter between the rider's legs.
The discs were an option (unfortunately an expensive and unpopular one - $130 was a fortune to pay for a kid's bike in 1972) for just 2 years out of the 5 year original production run on Krates ('68-'73), after that, as CK points out, Ralph Nader and the CPSC put their fears for the fertility of male American youth into legislation and the Stik-Shift bikes were no more.
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