on june 4, 1968, don drysdale threw his sixth consecutive shutout, blanking the pirates in los angeles and extending his consecutive inning scoreless streak towards his record of 58 and two-thirds innings. later that evening at the ambassador hotel, robert f. kennedy acknowledged big d in his speech following the california presidential primary. shortly thereafter, rfk was shot by sirhan sirhan. he died on june 6th.
the dodgers wore black armbands in honor of the former attorney general and senator, and this 1993 ted williams card of drysdale is the best example i can find.
here's a photo from getty images that shows the armband on both drysdale and his catcher, tom haller.