Would someone please explain the difference between 20 kopek obverse and 3 kopek obverse (modern USSR 1961-1991). They are of identical size but I just can't see the difference!
Also, is typical for 1 year only or they were messing up dies all those 30 years?

Thanks

P.S. There is a buzz about mules with obverse being reversed between those 2 coins

I’m not savvy on the topic of soviet coins. But from what I know it was a fairly common practice (not a mix up) to swap 20 and 3 kop dies, and probably 2 and 10.

On some years it is fairly easy to spot the difference. The 3 kop ribbons are more incuse, and the crest is a bit more elevated. But (imho) you’ll need a good die variety catalogue and a loupe to figure out more.