The latest letter is a different customer, or the same?
You can't discount the letter because it is a form letter. But if the signature doesn't match, I might be inclined to test the waters with a canned response, something like "We are sorry, but the signature on this request letter does not match that of any of our customers. To respect the privacy of our customers and prevent identity theft, unless a request can be validated we are unable to respond other than to request a valid query from a valid customer."
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell