United States Government Printing Office (GPO) Building No. 2

From www.gpo.gov:

GPO is the federal government’s primary centralized resource for gathering, cataloging, producing, providing, authenticating, and preserving published information in all its forms.

GPO is responsible for the production and distribution of information products and services for all three branches of the federal government.

Additionally, from The WPA Guide to Washington D.C. (1942):

Building 2, often calle dthe GPO Annex, completed in 1928, connects with the old building on the west side.