This is a knob that is designed for use with keyboards when the encoder sits with the key cluster. Usual knobs are tall and come in the way when typing. This is designed to avoid that.
There are two variants, the D shaped one is supposed to be pressure fitted on encoders. This is not recommended unless you know how to adjust dimensions for shrinking when printing, defintely not recommended for machining. The other variant uses a set screw and can be machined.
These are provided for non-commercial use only, and I don't take any responsibility for these if these do not work as expected.