The “Authorized List” that DCRA couldn’t find, has been found. The P ST billboards are indeed on the list, but the land is zoned as residential, and the list itself said that any billboards zoned as such were authorized for a time period “not to exceed three years from November 30, 1931.” (see below links) This is great evidence that the land was not, and is not, zoned for billboards: a violation of D.C. code.
Update on CCCA blog:
http://ccca-online.org/node/39