Samuelson's inscription (spelling correction):

ARE YOU THE FELLOW
Mr MELLON
THAT GRABED (GRABBED) ALL OUR
DOUGH. (?)
AINT (AIN'T) YOU BETTER UP AND
TELL US.(,)
WHERE IN HELL
DID IT GO.(?)

Interpretation:

"Andrew Mellon, banker and financier, is a friend only of the rich.  His sharp dealings in the first quarter of the 20th century took money out of the pockets of poor people like me (Samuelson) and put it into rich men's pockets and into industry.  

I'd like to know where the money I feel is rightfully mine went, Mr. Mellon, you dirty rotten scoundrel."

Send in your own interpretation.

Mellon, Andrew (William) 1855 -- 1937
Financier, public official, philanthropist; born in Pittsburgh, Pa. Taking over his father's banking house in 1882, he built a business empire by shrewdly anticipating growth industries. He helped found the Union Trust Company of Pittsburgh (1898), the Gulf Oil Corporation (1895), the Pittsburgh Coal Company (1899), the Aluminum Company of America, and the company that built the Panama Canal locks. A conservative Republican, as secretary of the treasury under presidents Warren Harding, Calvin Coolidge, and Herbert Hoover (1921--32), he stressed policies aimed at reducing the national debt. He drew fire by cutting income tax rates substantially as part of a tax-revision program to aid business and the wealthy. He forged agreements with European governments for repayment of their World War I debts and served as ambassador to Britain (1932--33). In 1913 he established the Mellon Institute for Industrial Research and he endowed the National Gallery of Art (1937).