FOUND one on a 66 Thunderbird today...Off Topic a bit but I also found an Early, original-style hood turn lamp harness push-in clip on the same trunk lid.
I came home with a small bucket filled with various mid-60's wiring fasteners...I'll start a thread on them. It might be nicer to have an old MPC to look up numbers but we might be able to figure out if any have Mustang applications from the pictures.