Okay, for those that are confused, Japanese Crystal has no data for the Odd Egg in the ROM, because Japanese players would go to the Pokemon Communication Center in Goldenrod City and download it off the internet via a cable that would connect a Gameboy to a cellphone. (The PCC was basically like a much bigger version of Goldenrod's Pokemon Center where you could do online stuff.) Back then in America, cell phones were mostly for adults, not for little kids, plans were expensive, (especially for something like internet access,) and there wasn't that much standardization so that's probably why it was a Japan-only thing.
Anyways, that's also how they got the GS Ball to catch Celebi back in the day, and I believe the PCC is still present in the Japanese Virtual Console version, it's just that the online services are inactive. However, you still get to go there and get the GS Ball after you beat the Elite Four, just like you can in the international Virtual Console releases.
But anyways, since the Japanese ROM does not actually come with the Odd Egg data, (unlike the international versions,) I'm not sure how they're doing that. Also, no one really knows for sure that it was 50%.
