Hi Ryan.
Going by what you've explained you may have a slightly different back end to mine. Mine has a simple single speed Crayford style 2" rotatable focuser and my 2" diagonal slips into the focuser 2" draw tube. As far as I know this is the focuser from factory. Maybe you have a newer or older series or maybe a previous owner has modified yours? Your primary mirror should be fixed and you shouldn't get any mirror shift.
By the sounds of it, it's probably just the thread size if your C11 visual back sort of starts to screw on. If you know the thread type of your C11 then you're half way home.
Unfortunately I know nothing about tech specs for telescopes, I just use them
