Ok, inverted horse or inverted surcharge...  depends on how you look at it, right?  

Some of the perforation varieties of the basic stamp are repeated on the surcharged stamps, but that isn't the interesting story behind these stamps.  

The story is that there was one sheet of Mirr 31 where the overprint was inverted.  This sheet was stored in the Philatelic Museum in Moscow and not in public hands.  A worker at the museum 'appropriated' them and brought them to the philatelic market.  He was caught and given a long prison sentence.   As part of the deal for reducing the sentence, he successfully recovered most (if not all) of the stolen stamps.  I don't know if he got his reduced prison sentence, but we do know that Moscow then sold those stamps itself for that same hard currency!   Surprisingly, the Scott catalog does list this error.  It is unclear if there were other sheets of inverts which were sold at the post offices, or if this is the complete source of this error.

For completeness' sake, here are the other three values of this set.