Chachi (Scott Baio), Joanie (Erin Moran), and both of their doppelganger dates, are wearing "The Kinks 65" t-shirts. The Kinks are a rock and roll band and were part of the "British Invasion" of music to America from England in the 1960s.
It was established in the previous episode "fonzie moves out" that fonzie lived with the cunninghams for six years, yet chochi says it was eight years ago he first sat on the cunningham couch with joanie. The chochie character was introduced long after fonzie moved in