Hello to all.
Yes I have check the remaining boards that I still have here, yes they all have it.
It looks like the mill didnt go right through the board. The top of the board has the slot milled,
from the rear side, there is a paper thick layer of wood left over.
I have taken a sharp cutter knife and could easly remove the remain wood.
Always push cut the blade into the slot from the rear side.
You can also use a Dremel.
Ill get the remaining boards fixed up before I send them out.
Sorry about this.
I wish all a pleasant Sunday.
Regards from Hamburg