The Manhattan address of apartment 12-B, the concealed base of operations used by Gary Seven in 1968, complete with the Beta 5 computer and a transporter hidden behind a vault door.
811 East 68th Street in New York City is the address of apartment 12-B, the office used by the missing agents 201 and 347 and then by Gary Seven in the Original Series episode Assignment: Earth. Outwardly an ordinary set of rooms, it hides advanced equipment supplied by Seven's unseen sponsors.
A bookcase swings aside to reveal the Beta 5 computer, and a walk-in safe conceals a transporter that works through a cloud of blue vapour and can reach McKinley Rocket Base. Roberta Lincoln arrives there expecting routine secretarial work and is drawn into Seven's mission, as is his cat, Isis.
Kirk and Spock trace Seven to the apartment and are briefly held up by the New York police; they return at the climax, when Seven uses the Beta 5 to detonate the orbital platform's warhead at a safe height.