The problem here is not the act of making or owning the shoulder stock. The problem is making or owning the shoulder stock without also having a 16" bbl'd reciver to attach to it. Without that, the ONLY thing that can be done with the stock is to make a short barrel rifle, end of story.
If a farmer buys a truckload of fertilizer and 500 gallons of diesel fuel, thats not a problem, since he has crops to fertilize and a tractor that runs on deisl fuel. OTOH if a 20 year old college student living in an apartment in the city buys a truckload of fertilizer and diesel fuel, thats constructive intent, and he is going to prison. "but I was going to buy a farm" just wouldn't work in that case.... Basically, the same thing applies here. You have a stock with no purpose other than to make a SBR....
If the OP has paperwork and a serial number from the gunsmith making the barrel, that MIGHT work, but I sure wouldn't want to be the first one to test it.... OTOH, if he just called up his gunsmith buddy and said "hey can you build me a 16" barrel for a mark II" or something similar, then he is definitely on the wrong side of the law. When talking NFA weapons, you dont get it first THEN fill out the paperwork. You dont get to take your suppressor home or mount your pistol upper on your rifle lower until AFTER you get your paperwork back from the ATF. Same thing here, you need to get the 16" barrel BEFORE you get the stock.
As far as comparing this to a contender and being able to swap barrels...there is an exeption on the contender and only the contender/encore and I don't think it wise to make up your own exception because they are "similar".
Actually no, The encor/contender ruling does NOT apply ONLY to those guns. Most ATF ruling are like supreme court rulings, which apply to ALL similar cases (e.g. AR-15's), not just the specific case they are ruling on. That ruling would definitely apply here, especialy this part:
However, the Court also explained that an NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel);
(The full rulling can be found at http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf
That means that by having the mark II, along with the stock in his house, without having the 16" barreled reciver, even if they are not attached to each other, he has made an NFA regulated firearm (a short barrel rifle) Basically, it is the same thing as owning an AR-15 lower, an AR-15 rifle/carbine stock, and an AR-15 pistol upper, but NOT owning an AR-15 rifle/carbine upper. Its the STOCK that gets you in trouble. Because since you dont have the rifle/carbine upper, the stock serves no other purpose than to manufacture a short barrel rifle. Get rid of the stock or get a rifle/carbine upper and you are back in compliance....