That letter from the ATF says possession of a stock and a barrel under 16" does not equate to possession of a short barreled rifle. It essentially kills the idea of constructive possession in some circumstances. It doesn't say anything that could imply converting an Encore or similar firearm originally assembled as a rifle to a pistol is legal without registration.
In the NFA a short barreled rifle is defined as a rifle with a barrel under 16", or a weapon made from a rifle if that weapon has a barrel under 16" or an OAL under 26".
crazymonkey's Encore was originally assembled as a rifle. Cutting the barrel to less than 16" or reducing the OAL to less than 26" would turn it into an SBR.
The letter you link explicitly address this:
"Nonetheless, if a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length is assembled or otherwise produced from a weapon originally assembled or produced only as a rifle, such a weapon is a "weapon
made from a rifle" as defined by 26 U.S.C. 5845(a)(4). Such a weapon would not be a "pistol" because the weapon was not originally designed, made, and intended to fire a projectile by one hand"
"Held further, a firearm, as defined by 26 U.S.C. 5845(a)(4), is made when a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length, is assembled or produced from a weapon originally assembled or produced only as a rifle. Such weapons must be registered and are subject to all requirements of the NFA."
