After years of being unable to afford one of these plastic viewers (the Tom Mix fans jack the price up too high for this humble KFO collector), I found one at a reasonable cost - though slightly more than the 20¢ it cost originally!
It was a premium offered in 1949, and came in the two styles seen below. It's very tiny - about 1¼ by ¾ inches. The picture discs are about the size of a contact lens, but seen with the viewer they're suprisingly clear and readable. The topics range from Tom Mix, Lou Costello, the Marx Brothers, Fighter Jets, a comic character named "Steve Screwloose," and of course, "Kukla, Fran and Ollie" (going on a picnic).
Here's the best I could do in blowing up the KFO disc: