It's a "back-of-the-book" issue, a postal tax stamp that benefited child welfare and was mandatory on mail during certain periods. It cost 1 centavo (fancy rendering in circle). This dates from 1929, Scott # RA8. There is a Scott-listed design variety. Both are minimum value. A near-similar design in brown (5c) or green (2c) also exist, I believe used for mail rates other than domestic first class.