The letter would not be sufficient in a court of law, that the rifle was assembled pre-ban, that would come in the form of testimony by a BM rep. or at least a notarized letter addressed to the court, not the owner(s), too much possibility for forgery. The letter's best use would be to prevent an agent of the law at the local level from seizing, vouchering as evidence, the rifle and arresting and holding, possibly for a night or two, the owner. But, if, "acting in good faith", the officer still did not believe the letter, he/she could detain the owner. Who, now, runs the risk of temporarily having his/her rifle seized and invoiced, thereby causing the owner to go through a multitude of legal exercises to, get his/her rifle back, hopefully w/o scratches etc.
That's why this entire ban, is a nightmare from the law enforcement, POV.
Should an individual be arrested, in poss. of a legal pre-ban, and charged w/ the post ban felony, even if acting in good faith, the officer, and municipality runs risk of lawsuit, as the gov. has not done an adequate job of making the serial # issue an accessible database for the leo to access. If the gov. makes the law, the resp. is on them to make it enforceable w/o undue hardship on the law abiding.
The sticking point becomes, ideas like"adequate" and "good faith" and "undue". Which are abstract, and would have to be weighed and measured by a court. LoL -S2