The difference is in name only, but credit unions often use a "master" signature card. It governs every account (transaction, savings and time) owned in the same right and capacity; i.e. the same owners, beneficiaires etc. It's a great time saver...
Examiners do not normally review contracts unless they incorporate federal requirements. I suggest that you treat it as a document granting signature authority and retain it for five years under BSA. While BSA does not require you to keep originals, your state law might.
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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.