Card Identification Forum

Asking for a friend who recently picked this card up. To both his and my knowledge all the other cards in the set have a set print run but no others save for this card have a serial number on the back. All the Print Runs range from 5-25 and he has I believe 36 cards out of 50 in the set yet only this particular /5 has a serial number. Does anyone know why this may be the case? Is it an error? Backdoored? any help would be greatly appreciated

Thanks