I asked John at Goldline brakes. His answer: the parts are long-gone except for maybe in salvage yards; he knows nothing about getting parts from SSBC; he suggests fabbing something to keep the pad down in the caliper, as long as it's for function and not for show.
This place says they have a 72 Plymouth Valiant with dual piston calipers; is that the same thing as what you're working on?
Marshall Auto Wreckers, Alberta, 403-328-3316,
info@marshallauto.com, they want $50C for each caliper. A bit steep for a clip, but if it's something you have to have ... ? If that's the right application, they might sell you the clip cheaper.