It has to be an SBS, as it originally had a stock.
But on a side note, there's no reason to ever make an AOW via a Form 1, as you're going to have to pay $200 to register it. It's only the subsequent transfer via Form 4s that's $5 when it comes to AOWs.
As you're incurring the $200 tax either way, it's smarter to do an SBS as then you can add a stock or whatever you wish to it later on, unlike with an AOW (unless of course SBSs aren't legal where you live).
"Our contract called for 16 cases of rifles and ammunition for $10,000 dollars, not a machine gun...........That is our present to the General"-Pike Bishop
“When fascism comes to America, it will be wrapped in the flag and carrying a cross.”