This is a small square collet block that uses an ER16 straight shank collet chuck (see link below).
Collet blocks are used for holding and indexing round parts for milling or drilling. While these blocks aren't as rigid or precise as a hardened and ground steel version, they are fine for low precision cross drilling, and filing, and finishing.
These blocks are designed to be used with this collet chuck: https://amzn.to/2XAaCrQ
The collet chuck is held in with three 1/4-20 set screws. You will need to tap the holes for the set screws.
I've included the original CAD files, feel free to use or modify them however you want.
I also designed a hexagon version which you can see at this link: https://www.thingiverse.com/thing:3679868