Professor Oak's Custom Ball

In Pokemon Gen I, after getting your first Pokemon and battling your rival, you must head towards Viridian City and pick up Oak's Parcel. When you deliver it to him, he says that it's the custom pokeball he ordered.

What is this mysterious ball?

Well, it's custom, leaving the original Pokeball out. And knowing the Pokemon crew only knew of the balls featured in Gen I to refer to, we can cross out the other Pokeballs. There was only one master ball, and Red ha(s,d) it. The great ball and ultra ball wouldn't be very custom, but I won't cross them out, since they aren't the original. I'll just mark them as "unlikely".

Who remembers the Orange Islands in the anime? Ash got the GS Ball from Professor Ivy (and lost a Brock) to deliver to Professor Oak. According to my research, the GS Ball ended up with Kurt. Maybe Oak ordered the GS Ball back because he figured something out.

But I'm pretty sure the Orange Islands episodes were released in the early 2000s. That would mean any Pokeballs featured in Gold & Silver (possibly Ruby & Sapphire) could be it. But the GS Ball seems like it's the best bet.

So my final theory is that it is the GS Ball. But hey, that's just a theory...

A not really complicated but interesting Pokeball theory!