Oh boy, Gniezno provisionals. I posted some info on these a while back.http://goscf.com/t/42878&SearchTerms=gniezno
I thought that this pretty well covered how to identify these. Yeh, right.
One of the contemporary experts on Poland, Leslaw Schmutz, published a 5-part series on these stamps in "Przeglad Filatelisticzny" in 2003/2004, in which he threw the numbers of the 10fen out the window & instead, concentrated on the SHAPES of the overprint, specifically the inside & outside characteristics of the "0".
He took the last part of the series to discuss & illustrate the various forgeries on the market & wrote in depth about a particular cover that another serious Polish stamp savant displayed as genuine. Leslaw took several pages giving his opinion as to why the stamps were forgeries.
The Union of Polish Philatelists, in it's wisdom, sat on the fence, made no decision, & let the two protagonists fight it out at five paces with tweezers drawn. Now, if two of today's serious Polish philatelists disagree, what hope do we mere stamp collectors have with this issue.
Here's my Gniezno page, still need that 5fen mint & 10fen typo forgery