This adapter is used to fit the guide camera head to a Skywatcher straight through finder scope:
https://www.teleskop-express.de/shop...er-scopes.html
You can also buy other adapters listed on the main MGEN page such as T2 to 1.25 nosepiece to use on an existing guidescope.
I like using the Skywatcher finderscope since it's light and easily transferred between different telescopes.