What were Hooverflags?

Answer 1

Hoover flags were empty pockets turned inside out.
People did this to show how poor they were during Hoover's presidency during the Great Depression.
They were supposed to be "flags for help."

Answer 2

an empty pocket turned inside out was called a "Hoover flag;" the decrepit shantytowns springing up around the country were called "Hoovervilles."

That is my answer from Washington, D.C.

Answer 3

Empty inside-out pockets.