The "Lucky Bolt" from Dia-Compe.
It has a couple of uses. It can replace the lever knarp of an upper gyro cable allowing you to shorten the single cable of the upper gyro cable.
It can also be used as a cable anchor on a U-brake with a regular straddle cable. Or, use two of them with a London Mod.
Body diameter (not including head) is approximately 7.45mm
Length (not including head) is approximately 9.75mm
Full length (including head) is approximatey 13.5mm