99.9% sure they are VOLVO calipers, normally used with TWO brake pipes, as each pair of opposite pistons fed seperately, has one channel been blocked off next to the existing brake pipe ?, or maybe volvo didn't use all four in actuation on all cars.
The GS is NOT GS but SG, nothing to do with godspeed.
The discs are cut down discs from a solid one piece that has had the bell cut off that's all.
tabetha