"PBS" stands for "Progressive Braking System" apparently.
It seems one pad was fixed and the cable caused the other pad to move inwards; presumably when it met the rim, that caused the whole calliper to rotate and press the fixed pad against the rim.
Obscure bit of history on eBay:
Weinmann PBS-300 brake caliper
Never used them but apparently they're awful. Still, nice to see some originality.