Help placing the new knot

I just cleaned up my deceased uncle-in-law's brush. I have a new knot ready to go. What will the knot he secure to?

Any advice would be greatly appreciated.

Looks like you would have to fill the handle with some material and secure that with epoxy/glue. After that cures, you will have a surface to work on.

I use cork to "build" a shelf for the knot to sit on. Fill around it with epoxy. Wait for it to cure. Then you can set the knot

Place the knot in the collar and then place it upside-down in a vice or book or some other contraption. Then fill the collar with epoxy, and be careful so as not to fill the threads. Once it cures, you will be able to screw the collar back onto the handle.

Scroll down a bit here for pictures (not mine)

That's a cool Rubberset 400. Good luck with the restoration.