Century of Progress is from the '34 World's Fair. Seymour PY-ZF, with an "e" value code.

Elk is a generic, Seymour code PA-JA, with an "f" value code. Colors other than R,W,BL are worth a little more.

Both designs are in the $4-8 value range, retail, in my experience. I don't have the yellow ELK in my collection and would offer $10 for it if you want to sell.