You probably won't like this answer, but. It will fit a milling machine or lathe with large enough tooling to hold it. This is a shop made long reach endmill holder for working in a deep hole or spot not reachable with conventional tooling. Problem with machinist tools is we often make our own as the job call for. To buy something like this would cost more that the time and material for a machinist to make. We make the tool and get on with the job. You can buy similar holders but unless you are in manufacturing and production on something of something that requires them, it is a complete waste of money to purchase specialty holders, if you can find the right one. I will send a few links to emphasize.

You probably won't like this answer, but. It will fit a milling machine or lathe with large enough tooling to hold it. This is a shop made long reach endmill holder for working in a deep hole or spot not reachable with conventional tooling. Problem with machinist tools is we often make our own as the job call for. To buy something like this would cost more that the time and material for a machinist to make. We make the tool and get on with the job. You can buy similar holders but unless you are in manufacturing and production on something of something that requires them, it is a complete waste of money to purchase specialty holders, if you can find the right one. I will send a few links to emphasize.

References:

https://www.ebay.com/p/Precision-Components-End-Mill-Extension-Lrx-750-9-Stk-...

Link 2, would need ordered form China at quite the price for a tool only needed once. Plus its in metric.

References:

https://www.aliexpress.com/item/SCGO-SIDE-lock-End-mill-extension-holder-SLD6...

Reason:
Another opinion was accepted instead.