Looks like this linen card DC-134 of the “Thrilling Finish of the Fifty Thousand Dollar Widener” was mailed in 1949 or 1948, with postmark and stamp.  There’s a splotch of age staining on the reverse.  The Widener Challenge Cup Handicap was first run in 1936, and after a bumpy ride (so to speak) wound up in 2001 when Hialeah itself was closed.  Grade: 3