This is the item in question. It is evidently the cornerstone plaque from an old State office building in Washington D.C.
The story goes that after WWII, my great grandfather moved to D.C. to find work, and became a milkman. On one of his routes, they were constructing an underground tunnel between the state office building and the senate building. He found this in some upturned ruble from the construction.
I have searched and searched for any sort of reference to it anywhere online, but to no avail. The writing on the face is a series of excerpts from George Washington's Farewell Address in the late 1700's. His signature? can be found in the bottom right hand corner of the plaque.
The current owner has contacted the smithsonian about a donation, but they don't seem to believe the story.
Please Reddit, this is my Hail Mary. Does this object look familiar to anybody?
I should also mention it is at least 50 pounds and about 29x21.5 inches.