Isnt this all about intent, or am I reading this wrong? Is this the same letter that was made public a month ago?
I was under the impression that as long as you buy it with the intent of using it as a brace, you can occasionally shoot it from the shoulder because there is no regulated way on how to hold or fire a pistol. For example, the feds cannot tell you to use two hands, or one hand to fire a hand gun, but if you were to mount a vertical foregrip on the picatinny rail, you were in the wrong and had to file the paper work for a sbr.
I thought it was along the same lines as a stripped lower receiver. When you are filling out the paperwork at a gun shop, it is stated as a rifle lower. But that same lower can be used as a pistol lower. And if your intent is to use it as a pistol, you have to declare it. But, if you buy it and register it as a rifle lower, you can build a pistol at your own risk.