In fact there were two settings of the overprints, and in a recent Rossica bulletin there was mentioned the possibility of a third.  Note the different sizes of the word "Posta" and the size of the "15" between these two stamps.

There were also a couple of errors where the overprint was inverted, or where the overprint was printed too high and the word "Posta" missed the stamp or was the only part of the overprint to hit the stamp.  There is also a third fiscal stamp with a similar overprint (one was sold at Cherrystone a few years back) but it isn't clear if it was a proof or postally issued).