Based on the internet in Google, my description is what I came up with.  I sent a image of the cover to my friend in Germany, who investigated what this item was.  He came up with the same purpose of thie fuel cover on a German Panther tank during WWII.  There is a impressed mark a "A"  and a "3" on the top section.  Indents also suggest that it was a cover.  If you discover a different purpose for this cap, please feel free to contact me with different information.  It is quite heavy 5 lbs  7 oz.  The dimentions are 6 inches diameter and 1 1/8 tall.