So I’m reading through the whole document linked in the OP. After an explanation of the criteria used in the form, there are examples provided of firearms in several different configurations and an evaluation according to the scoring standards. The second example is a pistol using an unmodified SBA3 brace. By default, it scored 8 points in section II and would be classified as being intended to be fired from the shoulder. The third example uses a Shockwave brace mounted to a KAK pistol receiver extension. They determined the KAK extension counts as “adjustable” for length of pull due to the multiple detent locations. It scored a 5 in section II. Guess the Shockwave is out too if you mounted it on the KAK extension.
I seems to me that the multiple point options for length of pull are what might hem up many configurations. If that portion of section III was limited to an either-or proposition of is LOP less than 13.5”, it wouldn’t automatically hammer so many people. That would also remove the possible additional points for mounting spacers or a Law Folder because they increase LOP.