'393' Counterstamp 1925 Stone Mountain Commemorative Half Dollar - Any info?

Does anyone have any information on this? I have had these before, but never one without the state abbreviation.
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Does anyone have any information on this? I have had these before, but never one without the state abbreviation.
Comments
It is a serial number (post mint), added by veterans groups.
Sometimes it increases the value.
2018 thread here
https://forums.collectors.com/discussion/998494/1925-stone-mountain-half-dollar-with-odd-stamping-on-reverse
2020 thread
https://forums.collectors.com/discussion/1039675/a-rare-stone-mountain-with-buy-it-price-2500
I counted 7 prior threads on these.
And right here on PCGS CoinFacts:
https://www.pcgs.com/coinfacts/coin/1925-50c-stone-mountain/9378