In the 1960's thru 80's, the NFL produced a limited number of transparency slides for use in television broadcasts. My source says that they only made one slide of each player each year, and only a couple of copies of each slide. I strive to be a John Hannah "super collector," so I pick these up when I can. Here are the ones I have so far:
1973 pre-RC, above
1977 and 81
1984 - the crappy scan doesn't do this one justice. In hand, the photo is colorful and just plain awesome.