Then again, the frequency on the coin page is slightly higher than the other years “only in sets” and most importantly, you seem not the first one to find it in circulation as there are a few for exchange in AU or without mentioning it being in a set. So there is always a possibility that there were some extras released into the wild.

Would be interesting to figure out if there are more people owning this coin in lower grades than UNC. Also the 10 cent of that year seems to be less common N#9895 (also only in sets) than this 50 cent.