As Brenda notes, in Part V (a) of the SAR you described your supporting documentation, every document you used to complete the SAR. I suggest you pull those documents together before your meeting, not in the heat of the moment. Make certain you have everything before you sit down. If the agent wants something and it is not in your stack, then you know you need a summons or some form of legal compulsion before you can deliver it. You don't have to think or make excuses, you just say that you did not use that document to complete the SAR.
Some smaller institutions that do not file large volumes of SARs put copies of all their supporting documentation in a sealed envelope with their file copy SAR. If they get a later request for documentation from law enforcement even someone unconnected with the orignal filing knows anything in the envelope is fair game. If its not there, they know they need to ask for some form of legal compulsion.
As internal controls go, you can't get much more basic than this, but it does work.
Personal opinion: Be willing to reconsider the completeness of your SAR in the judgment someone who is attempting to enforce the law. If you come to believe your filing was not complete or it did not list all the supporting documentation you actually used, be willing to both file an amendment and notify the agent that you have done so.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.