While the Tuvan government signed contracts while seeking permission, some "entrepreneurs" started making "Tuvan" stamps without any concern for permission...just by overprinting stamps of the former USSR. 

There was a period of doubt but now it is certain that none of these were officially sanctioned by Tuva.  They certainly were never sanctioned by Moscow. The first stamp in the above illustration was the first reported in the philatelic press.  A reasonable enough looking "provisional".  But as you see the overprints got sillier and sillier (English inscriptions, and who would make a sheet of stamps all with different 'face' values like those satellite stamps?).  There are plenty of covers with these stamps too, but they are almost all not backstamped and never went through the mailstream.  Some registered covers did go through, either because the postal clerk wasn't paying any attention, or perhaps with the use of a judicious gift to the accepting postal clerk.

I've seen such fantasy covers for most all of the overprint issues, and even fake cindies (they even made "First DayCovers!".  But I've never seen a cover with the semiOfficial stamps.  If you have one, please let me know.