On further inspection, I see other Spanish stamps defined as "private" Pedro
but I have no idea how the definition is applied.
http://cjoint.com/data/geizk7CUFU.htm


"Pedro"
Well, the Edifil Spanish catalog defines it literally as a "private
issue". I think that what they try to say is that it was intended for use
only inside the Congress/Senate and not by the rest of citizens in the
postal offices. Not available to general public. Correct me if I'm
mistaken.