What caliper is this

It does indeed look like a mid 80's item.It looks a bit like the one on my old 74Dart as well, and the ones I put on a customers 72 Barracuda a while ago.

There were two or three sizes, identifiable by the piston size,so measure yours.
There were also two different caliper brackets, for two different rotor diameters. I do not know if that makes a difference to the caliper.
In any case, it would be poor practice to run one new and one that sat-open-to-atmosphere-for-far-too-long, used one.
I guess you know that to install these on your Dart, you need all the matching parts from a 73 up A-body,right?