In Saudi Arabia the executioner often use a
one-handed traditional scimitar weighting about 3-4 lbs. There is no wooden block, the prisoner just kneels with his head low. (see non graphic image
here and
here)
Also according to reports
"normally it takes just one swing of the sword to sever the head" (from
here), but
"sometimes more than one stroke is needed to sever the head" (from
Amnesty).
So if a skilled executioner is employed then one blow is usually sufficient, even if there is no block holding the neck. Given the above information:
1) How many HPs would you give to neck?
2) How would you model the above in GURPS?