FinCEN used some language in its guidance on MRB's that I thought opened the door to using 314(b) as the justification for dropping a dime on a SAR subject:
To the extent the financial institution becomes aware that the marijuana-related business seeks to move to a second financial institution, FinCEN urges the first institution to use Section 314(b) voluntary information sharing (if it qualifies) to alert the second financial institution of potential illegal activity.
Assuming you have evidence of suspicious activity beyond a criminal record, I would still not suggest you use any tangible form of communication. A brief, very brief, fact based phone call to the other bank's 314(b) contact should do it.
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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.