Some Jewish tombstones display 2 hands arranged for the priestly blessing. Priests (Kohanim) are assumed to be descendants of Aaron, brother of Moses and the first Kohen. And, yes, this symbol is like the Vulcan hand greeting (“live long & prosper”), suggested by Leonard Nimoy (Mr. Spock), who saw the blessing in synagogue as a child. He modified it, as Vulcans use only 1 hand.
Leaving a penny at the grave means simply that you visited. A nickel indicates that you & the deceased trained at boot camp together,while a dime means you served with him in some capacity. By leaving a quarter at the grave, you are telling the family that you were with the solider when he was killed. According to tradition, the money left at graves in national cemeteries & state veterans cemeteries is eventually collected, & the funds are put toward maintaining the cemeteries.