This falls into the Class III catagory. A class 3 dealer I know sells them and he said you have to get the $200 stamp to own one.
like David said, $5 transfer tax on an AOW. for your information, had the receiver
been registered and equipped with a shoulder stock sometime during its
manufacture/birth, then yes, it would be a SBS and then the $200 transfer
would be applicable.
But in this specific case, Mark Serbu converted this from a Mossberg Maverick
which was released by Mossberg as a pistol-gripped-sans-shoulder-stocked
firearm, thus it is an AOW.
Note that Serbu also has created similar using Remington 870s which left the
Remington factory in both pistol gripped, as well as shoulder stocked firearms.
As such, the 'shorty' would be
AOW ($200 manuf tax....$5 transfer tax-from
the pistol gripped), or
SBS ($200 manufacturing and transfer tax-from the
shoulder stocked firearm).
You need not be a
Class 3 to transfer these. Use of that term is technically
incorrect as that is the SOT term for a dealer.
AOWs, SBSs, SBRs, MGs, SIs are properly referred to as
Title 2 firearms.
Title 1 firearms are those you and I normally find at local gunstores (garden
variety rifle, pistol shotguns).
Class 2 is the term used to decribe manufacturers of Title 2 firearms.