I was curious about the code on the bottom left of the ex-set, and I found that it was used in a some Japanese blogs and articles that it was used for a Feature phone service called Pokemon (Hiroba)Square.
game.watch.impress.co.jp/docs/20030731/poke.htm
According to the article, if enter the card code into the service, can get an illustration of the card’s wallpaper for Phone.
Does anyone know more about this or any other way to use serial code?
Very nice find! I’m afraid you already know more about the purpose of the ex era codes than I did. I’ve been aware of the different codes since I came back collecting in 2015, but despite having tried to find their purpose multiple times, I was never able to find anything.
Cool to finally have a bit more information, in that they could be used to show the card’s picture on the Pokémon Hiroba mobile app. 
Thanks for sharing! 
Greetz,
Quuador
The purpose of the codes is to make species collectors search far and wide to find the last code needed for the collection.
Yeah they mention that in the article.
So that’s what the codes were for. Maybe my phone didn’t do a good translation, but then why would there be multiple codes for the same card? Since the commons can have like 3 different codes. Wonder if this got planned for English, since they did the same thing.
Thanks so much for posting about this - I too had no idea what these were!
(other than of course adding pain to collectors)
I was under the understanding that different codes were either from different releases or different print runs. Different releases being like events or chain store promotions, etc
Take that with a grain of salt though, I have no idea where I heard that from
Huh. That would make sense. Wonder why it was only for the commons then. Weird how there isn’t much info on the codes.
Since 1st ed and unlimited have the same codes i kinda would doubt this, would be odd that they did the same amounts of prints of both 1st ed and unlimited for all the sets :E