Customer A: Principal, check 2(c)
Customer B: Attorney-in-fact, check 2(b)
If you have Customer A's identification from the time the account was opened, you may use that. However, you are only required to have identification for the person who conducted the transaction. If you do not have it for the person on whose behalf the transaction was conducted then you simply do not have it. Then, you check "unknown" for any missing information in a critical field.
As an aside, it was a significant judgment call to cash a check endorsed only by a fiduciary acting in that capacity. Fiduciaries are responsible for accounting for their receipts and they cannot do that if they cash a check payable to the person on whose behalf they are acting. It's also unlikely that the POA says the attorney-in-fact can cash checks. If you had contact with the principal and he confirmed the request, that would improve your position...
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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.