I love things like this , i believe i have found the stamp that it refers to , look here at the bottom of the page http://www.junior-philatelists.com/USStampsHistory26.htm
It is apparently referring to the Battle Of White Plains .
but what do you have? Country Ware does produce some Pewter plates , are you sure it is aluminum ?
with it being metal of some sort i believe i would get a picture of it (your link isnt working for me either) and post it on some of the relic hunter forums , google that term to find them .
it may be well worth a little research to price the item right .