Pokemon Theory: Professor Oak's Custom Pokeball

In Pokemon Red, Blue, Yellow, Green, FireRed, and LeafGreen, you must deliver Professor Oak's parcel to progress through the game. When you deliver the parcel, he says that it's his custom Pokeball that he ordered. But what is the Pokeball?
Well, firstly, it's custom, so the classic Pokeball is out. Great Ball and Ultra Ball are possibilities, but that's just if I can't find a better answer, as you can buy them at most Kanto stores, so Oak wouldn't have to ordered them. He's smart. Unlike some professors. *cough* Elm *cough*.
The Master Ball would seem like a possibility. But remember, Silph Co. only made one, and the president of Silph gave it to you. So the Master Ball isn't a possibility.
Is there any chance that it's the GS Ball? Remember in the anime, Professor Oak sent Ash to Professor Ivy to get the GS Ball. According to my research, it ended up with Kurt. Maybe Professor Oak discovered something or simply wanted it back.
Yes, there are many Pokeballs. But in Kanto, only the Pokeball, Great, Ultra, and Master Balls are available.
So as my final answer, my theory about Oak's custom Pokeball's answer is..............

The GS Ball!