Hello Ra'Jiska
You are correct. I have only one serial code "Mount & Blade: Warband - Napoleonic Wars"
Although I did order after the free play was finished the sale is on for another two hours.
I will wait for a while and contact steam support for the missing key.
The link you provided for serial number check does not like my forum login for some reason.
https://www.taleworlds.com/en/Account/UserGames
Standing by
