This holder is designed for use with Dillon 650/750, as it attaches to the case/bullet feeder rod and is intended to hold “Handheld case for pushbutton”, published here on PrusaPrinters.
The source model is published as well, so you can modify the attachment according to your needs.
Printed in PLA, use any material of choice, no supports, simple and straightforward print.
The author hasn't provided the model origin yet.