Rio Muni

Scott No.: 44, 46
Date of Issue: November 23, 1964
Species:  Greshoff's Tree Frog (Cryptothylax greshoffi)
Bird Species:  Helmet Guineafowl 

These stamps were issued for Dia Del Sello--Postal Stamp Day--which was first celebrated in 1935 in Austria.  The Wikipedia translation from Spanish wasn't perfect, but I gleaned that it might also have something to do with postmarks.