A circular slide rule
A flight computer was a type of manual, hand-held calculation tool, similar to a slide rule.
This device was sometimes used on board the USS Enterprise during the 2260s. (TOS: "Mudd's Women", "The Naked Time")
A operations division crewwoman was reading a flight computer as she walked through a corridor aboard the Enterprise in 2266. Around the same time, Bobby walked onto the bridge reading one briefly. (TOS: "The Corbomite Maneuver")
Spock later made a number of calculations on one in 2268, while the ship was held in place by a big green hand. (TOS: "Who Mourns for Adonais?")
This "real world" tool is in actuality an avionics device.