How long (in inches) does a pistol grip have to be before the AWB applies?

CRC

I dont know, and do not think there is a defined length (or angle for that matter). That is why it is worded as "conspiciously", the law has been left intentionally open for intrepretation. IMO: If the trigger hand fastens to a OPEN pertrusion outside (or from) the stock, its a "no-no".
Thumbholes have tradionally been iffy, and at the moment know of no manufacturer who has used a thumbhole stock to be able to add another evil feature (voiding the pistol grip).
Note the addition of the loop to the new civialian Augs (USR) even though they lack the +1 evil feature.

2 different laws at work here:

the 'thumbhole stock' has always been considerd a pistol grip under the AW ban (922(v)).

However, a thumbhole stock is not a "military feature" for purposes of the import ban (922(r)).

The augs have a thumbhole stock because they couldn't be imported with a separate pistol grip.  However, as far as the AW ban and evil features are concerned, the thumbhole stock is still a pistol grip, and counts as the one legal feature on a postban semiauto with a detachable magazine.