Polish 50 groszy 1990-2016 N#521 was redesigned in 2010/2011
in 2010 we can find two variants of revers
the old one - like for 1990-2009 - see also the picture for N#521
(the picture below is from 2010 coin)
the new one(the picture below is from 2010 coin):
The new one was used for 2011-2016 coins
The new one looks like revers of N#102872 and N#23381
Main differences you can see in word GROSZY
Old one: R with small whole, O thick - with changing thickness
New one R with large whole, O thin - regular
PS. for other nominals thare were reported some coins from 2011-2013 with old
revers. But I have found bo confirmation for 50 groszy.
PS. 2 some sources reported that 2010 desing is new, however differs a bit from 2011+ new designs -
but I was not able to confirm it (see) so far.