As mentioned above, they tried to make them cheaper by changing the
receiver metal. The finish wears, gets rust freckles, and generally doesn't
hold up well.
I sort of suspect that your gun has been refinished. The edges on the
receiver look a little soft, and some of the screw holes look a touch
dished. One of the other attributes of the "new and improved" '64
metal was that it doesn't re-blue well. Usually comes out some shade of
gray or purple. Your finish is pretty even over the whole receiver, I'm
thinking it was polished out and tanked, with poor results.