This is a dual helix flexible coupling. The bore sizes on the STL are 6mm and .375". The length of the coupling is 2.35".
This is a plastic variant of coupling found on Sterling Drive Products coupling catalog

I printed this coupling with the following parameters
Printer: TOM
Firmware: Jetty Sailfish
Filament: ABS 1.75mm
Fill: 100%
Extruder temp: 230C
Bed temp: 115
Print speed 60mm sec
Travel speed 80 mm sec

The STL file already has supports built in so there is no need to add additions
Print the coupling and the supports are easily cut using a hobby knife

Clean up and use as desired

I am using this design on Z axis stepper to leadscrew coupling on a 3d printer that I designed and am in process of building

I have also included my native inventor file and Step file if you wish to do some changes

Just for a note I printed the first one with a higher temperature and it turned out it was very brittle so do some testing on your temperatures.