I'm also curious if there's any guidance on just how "fixed" a stock needs to be. Does something readily adjustable with hand tools qualify?
Also note that Mass. law basically just incorporates the old federal language, which refers to "a folding or telescoping stock", but that the new versions of the AWB that have been floated for the last few years include a much more inclusive definition: "The term `folding or telescoping stock' means a stock that folds, telescopes, or otherwise operates to reduce the length, size, or any other dimension, or otherwise enhances the concealability, of a firearm", which (along with a bunch of other rather expansive definitions) would likely be a rude surprise for Mr. Fudd.
I've been contemplating the legality of putting a target stock (with adjustable length of pull) on a post-ban AR-15. Seems to me that as long as it's not telescoping or folding it's OK under the current rules, but if anyone sees things different I'd be very interested to hear about it.