Considering all available evidence, I think it's very reasonable to assume that the democrats filled out the official subpoena forms and included them with all of the subpoena letters they sent out in regards to the impeachment inquiry, even before October 31.
I understand that the only evidence that you are willing to consider is whether or not you are able to see a picture of the actual subpoena, and think it's reasonable to assume that the democrats didn't fill out the official subpoena forms and include them with the subpoena letters.
What is "all available evidence?" You mean some people talked about it? People talk about lots of things that don't exist, the Trump Ukraine quid pro quo for example. Your assumptions are not a standard of evidence, neither are the words of others.
It is very reasonable to assume you can not produce the subpoenas because they never existed. A filled out subpoena form is not a subpoena. An authorized, signed, and recorded subpoena complying with due process is a subpoena.