What is a subpoena, if not a signed letter from an agent of congress directing either testimony or production of information? Its possible this letter in itself could be considered a subpoena, since it was delivered in an official capacity. The only formalisms I'm aware of are guidelines and convention, which don't really mean anything anymore, and this letter seems to fit all the definitions I can find. I know it's a dumb question to get hung up on since obviously "talk to a lawyer" is the #1 thing to do here, but still it's an interesting question as to how legally binding an order in a form like this actually is.