The difference between a Rem 870 that is registered as an AOW and a Rem 870 that is registered as an SBS (Short Barreled Shotgun) is that the AOW can not have (and can never have had) a stock attached. AOW’s are not shoulder fired weapons. If you attach an 870 AOW to an AR-15 then the buttstock of the AR-15 is now the buttstock of the 870 and you now have an unregistered SBS. If you want a Masterkey you need to use an SBS not an AOW. An SBS can have a stock or no stock.
As far as NFA tax goes, it costs $200 for you as an individual to Form 1 (manufacture) an AOW or an SBS. Once registered, any future transfer of the AOW (on a Form 4) is $5 while transfers of the SBS are still $200. You can send your regular old 870 to an SOT and he can convert it to an AOW (tax free) and then transfer it to you for $5.
Personally, I would pass on the Masterkey. Looks cool, but too heavy and throws the balance off. Just carry an AOW slung on your back.