What caused inboard front disc brake pad to score the rotor?

OK when you say shim are you talking literally about a very thin pice of spring steel
or the metal brake pad backing?

that looks like the brake pad backing has come off the pad..

brake pads have a backing with keyed ears that fit into the groove either side of the flaoting caliper or if its a KH i think they have pins through...so am assuming this is the claw bracket floating 1 pot caliper that takes pads like these Fits Dodge / Mopar 1973-83 Various Models New Brake Pads Raybestos D123 D86 D269 | eBay

in which case 1 pad has a Z shaped spring to keep it in place and 1 as you can see has little ears on it
the hole lot is held togther by 2 plates that press the cliper and its pads into the bracket
suspect either detached pad from steel backing or you have something missing