There are two letters Y (actually a "you" sort of sound) on the coins. One is the first letter of the name of the country in the text "national bank of Ukraine". The other is the second word in the legend on the other side. Taking the two variants on the site Christian linked to, you get four occurrences. Three are the same. The fourth is the first character of the word Ukraine on the coin described as "Ukrainian letter Y looks similar to Y/2".
Concentrate on the little loop left below on this letter. From a distance, the loop looks firmly closed on the commemorative side, weakly closed on the arms/bank side. Use the zoom tool and you will find the "difference" to be microscopic.
Peter