This is the coupling part of an RC axle coupling.
Other side of coupling Fits for 608 Bearing (not too tight)
The rubber coupler will fit inside just fine and will be able to turn minimum 19 500rpm in balance if the fixture is stable (As far as my tests went).
You can customize the .f3d file - for example to open in Fusion360
Modellauto-, Boot Kupplung CAD
Passend zur Kupplung vom Conrad.