Whoever is selling it made a mistake. April 18th the Dodgers played at Philly. May 18th they played at St Louis so this ticket has nothing to do with the Dodgers.
As others have said it looks like Philly's May 18th game against the Reds was rained out and made up on June 18th as a doubleheader. Look at the schedules:
Brooklyn:
http://www.baseball-almanac.com/team...p?y=1950&t=BRO
Philly:
http://www.baseball-almanac.com/team...p?y=1950&t=PHI