What is this harness for?

Don't think the seatbelt interlock would be a separate harness. At least on my '74 it's part of the main harness and the wires going to the starter relay are yellow, don't see any yellow wires on this one.

The relay box doesn't look right for the seatbelt interlock either. Mine stands up more off the inner fender. But if it was, it should have a red cylinder sticking out of it with a button in the middle that can be pushed with a pen or something. That would be on the back of the relay, so it could be there since we can't see that side in this picture.

I'd bet H22A4's first guess is correct for the whole thing. It's everything that is needed for the fuel miser option.