Cisco gives Caitlin a copy of the "Keep Calm and Han Shot First" shirt he wore on his first day at S.T.A.R. Labs, which was seen in the flashbacks in The Sound and the Fury (2015).
Caitlin sends Barry to a van parked outside Ferris Airfield registered to a "Carrie Bates." While Ferris has been mentioned multiple times in the Arrowverse (as a reference to Green Lantern Hal Jordan), Cary Bates is a long time DC Comics writer who had a long run on The Flash comic book (as well as Superman and Justice League) from the late 1970s to 1985. Additionally Bates created the original Rainbow Raider character. (Bates also wrote himself into a couple of Flash stories.)
Cisco looks at the photo of Team Flash that was taken in "The Sound and the Fury".