A letter from a Deputy DA at the state level is about the worst sort of "protection" imaginable. His opinion, that's all. When he leaves office for private practice, that'll change. And there are far more DAs at the local, county and municipal levels that would disagree and charge you anyway.
I think this is VERY dangerous to be flippantly telling someone they can have a flash hider AND an M1A at the same time. Letter of the law, maybe, but we have now seen how California DOJ operates contrary to the letter of the law. Constructive Possession maybe not specifically legislated or regulated at the state level for firearms, but there are accompanying violations like Conspiracy or Attempt that can easily be charged by even innocently having the two parts together in the wrong place in the wrong company. A DA need only believe he can prove it. Allowing someone to believe they can have both a flash hider AND a non-AW M1A like there was nothing to it is a bit irresponsible, IMHO.
City of Los Angeles, by the way, DOES have a Constructive Possession law for AWs, so be forwarned if you live here OR are passing through with your M1A and detached flash hider. LA Municipal Code 55-05 (1) 4: Any part or combination of parts designed or intended to convert a weapon into an Assault Weapon (IS an Assault Weapon). In LA city, this situation is illegal. State firearms pre-emption or not, LA will hammer you.