Thanks and welcome to CCF.
It's an inverted overprint error, minus the serial numbers, but would need in-hand inspection to determine if it's genuine and not spurious. Sometimes the overprint error is right side up and sometimes it's inverted (upside down). Most of the times it will include the serial numbers, but not always.
Wait a minute. I took a second look and failed to see it's got a complete overprint also on the front. I've never seen that before. It's definitely gonna need a closer inspection to determine if it's real.
Edited by SteveInTampa
09/18/2021 7:13 pm