Yup! Thanks, that explains some behavior I saw when playing around with waypoint editing, too.
I did note that the markpoints started at 0 for me when hitting the EHSD MK button, counted up to 10, then went to 1, then 10 again, as I hit the button repeatedly. On a three-waypoint flight plan, the next waypoint I could enter manually was 24. (Which would make sense if there were 0-10=11 plus 1-10=10 markpoints in the computer.) I also saw the same thing as you, where the markpoint number displayed in between the up and down arrows was always MK0.