On some cars you will find one of each, one for the front and the other for the rear. it depended on the amount of plate space and as mike stated, some times covered too much of the plate.
BTW, phone numbers, 800 numbers, mutli brands, . coms, ect did not come till much later. I have never seen a 60's or 70's frame with a casting date.
I still have not found ANYBODY that remembers TED GREEN Chevrolet in Torrance CA., where my car came from. I live about 5 miles from that dealership (now Martin Chevrolet and Geo)
And have asked many many long time local cruisers to no avail. I was only 6 in 1968. Just makes it sweeter when I do find some plate frames.