What unown letters are available through the Aug. 2017 European event?
I live in one of the cities where the event takes place, and I've only seen EUROP.
I've seen a screenshot of someone catching another letter (which was the first letter of the city's name), but I'm not sure if that's coincidence. If it's not, then at least those are way rarer than the other letters.
In order of rarity the letters have been occurring roughly as follows for me: E, U, P, O, R. It's not a big difference in rarity, but it's clear that E is much more common than R.