#H3; a larger flush mounted nut as used on the Helle Lappland. The OD's are .627" and .412". The inside diameter of the hole is .192", which is too large for many of the blades. It can be made to work, but the tang isn't likely to completely fill the hole without a considerable amount of peening. It will work very nicely with the Roselli leuku blades.
This is for one End Nut.