This is a mock up (or as a cool teacher I used to work with would say:

'A first best effort') of a Jig to hold a Japanese blade.

WHY?

Well of course I freehand sharpen - but when you get an old one....

That has a 23 degree bevel on it and you want to change it...

This just clamps into the honing guide.

Once you have enough flat you can stop using the jig of course.