For your level of expertise on subpoenas it's surprising you still think that the dems were using a cover letter as the actual subpoena. They aren't. Those letters are included with the actual subpoena to define scope.
Just because it's public record doesn't mean it's on the internet. You could request a copy of the subpoena by making a FOIA request with the state department which takes a couple months. I don't really see the point though, it won't say anything that's not included in the letter that was sent with the subpoena.
Cover letters include a description of contents, usually including page numbers, that is the purpose of a cover letter. Also they are usually labeled as such. Now your strategy is to downgrade this from a subpoena to a cover letter? Just admit it was a lie and you believed it. If you are so sure I am wrong produce the subpoena. A FOIA is not required for published, public documents, as the vast majority of court records are. How many more excuses can you think up to justify the fact you cant produce the actual subpoena?