On October 9, 1998 Spain issued this 70 peseta stamp (Sc #2960) to publicize World Stamp Day. The stamp depicts the UPU logo imbedded in (purportedly) the log of World Association for the Development of Philately (W.A.D.P.).
From Wikipedia, "The UPU in conjunction with the World Association for the Development of Philately, developed the WADP numbering system. It was launched on 1 January 2002. The website displays entries for 160 countries and issuing postal entities, with over 25,000 stamps registered since 2002. Many of them have images , which generally remain copyrighted by the issuing country, but the UPU and WADP permit them to be downloaded."