Interesting, Will. I have tried to clean the triangular sections left after the boring machine with my common joinery axe, it is not a practice I continue. The axe head is too short. I can imagine a design which would do this, a longer head that would reach into the mortice without banging ones knuckles on the surface of the timber. The morticing axe as pictured in Miles Lewis's collection almost fit my thoughts. But I see a slick mounted in such a manner to resemble a axe. I just go back to my slick it works very well for me.

I could go along with the fence theory.

Nice pdf from Miles Lewis.

Tim