A slim device resembling a fountain pen carried by the agent Gary Seven in 1968, able to stun, kill, open locks and operate the hidden equipment in his New York office.
The servo is the pocket tool and weapon used by Gary Seven in the original series episode Assignment: Earth. It looks like an ordinary silver pen, with a small antenna that extends when it is in use.
Seven uses it to put people gently to sleep, including police officers and guards at McKinley Rocket Base, to open doors, to reveal the vault that conceals his transporter, and to neutralise the Enterprise's force field when he escapes from the brig.
Near the end of the episode his secretary Roberta Lincoln picks the servo up and holds him at bay while he tries to detonate an orbital warhead safely. Seven warns her that it is set to kill, and she has to decide in seconds whether to trust him.