Okay, so I am not an expert... but when I worked down south (Colorado way), we had several credit unions that would bank with us. If we ever had anything come back on them for endorsement issues, we sent them back to the CU. We never had any way of proving that the account belonged to the payee. That was their job. They are responsible for the endorsement of the item. Hope this helps some.
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...it's all opinion, until proven otherwise...