The National Postal Museum opened to the public July 30, 1993. The museum houses and displays the national philatelic and postal history collection which is the largest and most comprehensive of its kind in the world. The museum has five exhibit galleries, a Discovery Center, and a Library Research Center.  More - https://siarchives.si.edu/collections/siris_sic_2239 and https://siarchives.si.edu/collections/siris_sic_1576.