Opps...sorry OPH didnt see the link already posted....
My main "stuff" hauler over the years (out of PA) also hauled for a collector in NJ or NY state who collected Muffler Men.
Said he had a estate property full of them.
He was a retired truck driver hauling with his wife seeing the Country until he passed.
He knew where many of these were located and would try and buy on the spot for his deep pocket customer up North if he ran across one.
This site has many of the locations and explains the variations and what they hold...;
(site also has other strange roadside attractions by State)
http://www.roadsideamerica.com/muffler/
PS: buried on that busy site is story on their origins...thought it might be interesting for some...;
http://www.roadsideamerica.com/muffler/origin.html