How embarrassing... When I went there in 2015 I didn't notice the cornerstone! Probably because it was around the corner. Here it is from Google Street View, 2018, not very legible... On top it says Henry Morgenthau Jr, Secretary of the Treasury; James A Farley, Postmaster General; then two more names and titles, and the year, 1935. It is probably the same as the (former) Bronx General Post Office cornerstone.

The New Deal in NYC 1932-1943 | Frank da Cruz | fdc@columbia.edu