At 3:30 min the dry erase board shows rage killing as an idea for the Seinfeld reunion, but this only occurs later as a flashback, so the incident had not yet been told at that point.
In the flashback scene set in 1962, Madeline and her husband listen to and sing along with "Don't Say Nothing Bad About My Baby" (The Cookies), which was not released until the following year, 1963.