Topic: UPU (the Universal Postal Union)

The following Polish postage issues commemorate the Universal Postal Union:

Fischer 498-500 (75th anniversary in 1949)
Fischer 1860 (Opening of the new UPU building in Bern, Switzerland in 1970)
Fischer 2160 (100th anniversary in 1974)
Fischer 3645 (125th anniversary in 1999)