The only 590a1 SBS model number I know is 52689-0. It's the 14" with the GRS and Speed feed stock. As for the tax stamp, yes the $200 is in addition to the cost of the firearm.
Your best bet is to order direct from Mossberg. They are made by special order and there's a 5-6 week wait time for production. First you'll have to find a local dealer with a Class III license that is willing to accept the transfer. Then, buy the shotgun from Mossberg and have it sent to the dealer. Once you have the serial number you can fill out an NFA Form 4 for the transfer. You'll need your fingerprints, passport photos, your county Sheriff's approval, and a $200 check for the tax stamp before you send off your paperwork. It'll likely take 6-8 months for your paperwork to come back and if you're approved then you can take it home.
Another option is to buy an 18.5" 590a1 and either have the barrel cut to 14" or buy a factory 14" barrel from Mossberg by special order. This would first require filling out an NFA Form 1, jumping through the rest of the hoops above, plus the $200 tax stamp and the 6-8 month wait. If approved, you can have the barrel cut down or instal a factory 14" barrel...
