Interesting coin. I will not contradict John him have more experience then me in errors or varieties.
On your coin me I see some interesting points, which could not be after striking damage:
1. E from REX. is longed and broke at the base.
2. A from GRA touch the dot.
3. The CA we see it is negative printing? this is questionable, how?
4. The other negative "21" I have no explanation.
Before to answer you I look all my 10 1921 and no one show something close. I thing must be investigate more. Could be a strike. In that time was no automatic strikes, so move or fast done could happened.
Me I consider a interesting coin for those who look for errors and varieties.