Charlie, we 124 guys keep this under our hats but MB cheaped out and put single piston calipers on 6-cylinder 124 in place of opposing piston calipers on 123s and 126s. It looks like the floating caliper on your Datsun that has to come off to change pads. You can't just push out a couple of pins, slip in new pads and have time to make a sandwich during a commercial break. V8 124s get proper opposing pistons calipers with easy-to-replace pads.

EPC calls the bracket a brake anchor 000 420 12 15 left, 000 420 13 15 right, including all hardware, except the brake cylinder housing (caliper) and the bolts that hold the brake cylinder housing to the anchor. The anchor is common to early 124s and possibly all 6-cylinder 124s (hint, used isn't a bad way to go). The anchor lists for $210, discounts to $150. There's also a 'cup' parts kit 001 420 13 83 that looks like the slidy accordion thing and a bolt. The brake anchor includes this cup. The cup alone lists for $23.50, discounts to $16.50. Maybe this cup is what you call a socket.

Sixto
87 300D^2