WHICH ACCOUNT WAS THAT?

I received a letter from a credit card company stating that it was making a change to my account. The company however, did not identify itself, so I did not know which credit card the change pertained to. I wrote a letter to the return address on the envelope telling them that, if they are going to send me mail they need to identify themselves, so that I would know which card they were talking about. They wrote me back, "We regret, we cannot answer your questions without an account number"!!