This is not a short barreled shotgun. An SBS is intended to be fired from the shoulder. A shotgun with short barrels and a pistol grip, like the one in post 5, is classed as 'Any Other Weapon,' AOW.
Since you are doing gunsmith work, I'll bet you also did not know that it is illegal for an FFL dealer to attach a pistol grip to a shotgun for a person under the age of 21, regardless of the overall length. The reason is that this gun is not intended to be fired from the shoulder. To be classed as a long gun, legal for sale to persons under the age of 21, the gun must be intended to be fired from the shoulder.
In an amusing example of stupid federal bureaucracy, you can legally sell the gun to an 18 year old, and you can legally sell the pistol grip to an 18 year old at the same time, as long as the gun has a shoulder stock affixed to it. The 18 year old can go in the parking lot and legally attach the pistol grip. You just can't sell it unless the shoulder stock is on the gun. This is addressed in the yellow book from the BATFE.
In common usage and in NFA regulations (I think) but the actual law (Title 18 section 921 (a) (6)) defines the "less than 26" overall" weapon as a short barreled shotgun.