I replaced a non-operational C/D player in my 73 with an original AM/FM radio. I'm guessing the "installer" used self tapping screws to hold the escutcheon plate on but it looks to me like the screw holes are threaded. What size do I need?

You are correct, they are threaded. They are 5mm x 0.8mm threads. They are not likely in very good shape if sheet metal screws were use, I'de chase them with a tap first and cross my fingers. I think they are trapped nuts if I'm not mistaken, may technically be replaceable if you "had" to... If not, drill and tap out to 6mmx 1.00 threads as a first go, or fill with an epoxy putty and re-drill and tap with the correct 5mmx0.8mm.