This is called a 'micro stop countersink' or at least part of one. It will fit in most all drills of acceptable capacity, milling machines, Lathes, etc. We machinists use them to deburr and countersink holes so screws mount flush. Judging by the quality of knurling, machined flat, and hand engraved size reference, I would almost put money on private shop made. This is because it looks like someone took pride in the craftsmanship. It is missing the bottom cage, but anybody interested in this kind of equipment can easily produce one. They are used a lot in the aerospace industry because this kind of countersink has a tenancy to chatter and leave undesirable finish. This tool minimizes that chatter.

This is called a 'micro stop countersink' or at least part of one. It will fit in most all drills of acceptable capacity, milling machines, Lathes, etc. We machinists use them to deburr and countersink holes so screws mount flush. Judging by the quality of knurling, machined flat, and hand engraved size reference, I would almost put money on private shop made. This is because it looks like someone took pride in the craftsmanship. It is missing the bottom cage, but anybody interested in this kind of equipment can easily produce one. They are used a lot in the aerospace industry because this kind of countersink has a tenancy to chatter and leave undesirable finish. This tool minimizes that chatter.

References:

https://www.google.com/search?q=countersink&tbm=isch&tbs=rimg:CarOs9_1OcPz0Ij...