IANAL however I did read this letter in it;s entirety and it appears to say that anybody who interpreted the AFT letter regarding the "intermittent" usage of using a pistol brace as a shoulder stock as "redesigning" the brace and thus creating an NFA firearm (can you spell FELONY) was incorrect.
If you do not alter the design of the brace in order to use it as a shoulder stock (e.g. remove the arm strap) then it is not an NFA firearm (i.e. SBR) EVEN IF YOU PUT THE BRACE TO YOUR SHOULDER.
http://www.thefirearmblog.com/blog/...ent=2017-04-29&utm_campaign=Weekly+Newsletter
If you do not alter the design of the brace in order to use it as a shoulder stock (e.g. remove the arm strap) then it is not an NFA firearm (i.e. SBR) EVEN IF YOU PUT THE BRACE TO YOUR SHOULDER.
http://www.thefirearmblog.com/blog/...ent=2017-04-29&utm_campaign=Weekly+Newsletter