And in the latest episode, "My Old Kentucky Home", the Zildjian crash cymbal used at the garden party didn't come into existence until the mid-1980's.
Zonks like these can be most easily splained away with "Forget it, Jake; it's Toobworld." This isn't the real world, so sometimes the same rules don't apply for the TV universe. To remain in the main Toobworld, they do have to share the same President as the real world - in this case, John F. Kennedy because of the timeline (for another few months, anyway).
But there's no Sterling-Cooper in the real world; none of these characters are real; it wasn't the Schillingers who ran the Utz company; Jimmy Barrett never got his own network TV show in the real world because he doesn't exist. Allowances have to be made, and the dictionary set and the cymbal have to be included as exceptions.
In Toobworld, both of those items did exist back in 1963. No Zonk.