Most calipers use square seals, most caliper also use a polished piston and a seal that sits in the bore, not the piston. A much better design. Commonly those seals are all square. The only caliper i know of and have ever taken apart that is different is the corvette junk.
Thats my understanding also. I just can't figure out why people use o-rings in Corvette calipers instead of quad rings. Also, an o-ring is less compliant than a quad ring which is less compliant than a lip seal.
I've never taken a Corvette caliper apart, maybe I am missing something. Couldn't new sleeves be made that retained the seal (instead of the piston retaining the seal)?