When this handout was originally distributed (July 1967), used typewriter platens were a hot commodity. It was a common practice back then to sell resurfaced platens, but of course that meant suppliers needed a supply of old platen cores to resurface. It's interesting to see which platen cores were in greatest demand back in the '60s, and to compare the differences in the designs of these various platens.

Equally intriguing is that I've never heard of some of the brands listed; it's posible that some of those manufacturers didn't produce typewriters, but other machines such as adding machines that also used platens.