I emailed the ATF, and this is what they sent me -
The Benelli Supernova shotgun: When equipped with the standard buttstock or the fixed pistol-grip buttstock assembly, contains the following parts from the list presented above:
4. Bolt carrier.
5. Trigger housing.
12. Magazine body.
With regard to the fixed pistol-grip stock of the Benelli SuperNova shotgun, ATF has determined that this assembly consists of a buttstock only. This assembly has an integral pistol grip molded-in, and is designed to accept a “rubber grip” accessory. This accessory acts only to provide a comfortable gripping surface and is not considered a “pistol grip.” Please note, however, that the Benelli SuperNova telescoping stock incorporates both a buttstock and a pistol grip. The pistol grip of this stock assembly is designed to accept a “rubber grip” accessory. This accessory acts only to provide a comfortable gripping surface and is not considered a “pistol grip.”