If you explore the realms of Holodrum and Labrynna in The Legend of Zelda: Oracle of Seasons and Oracle of Ages, you'll eventually find a special shop that sells exclusive magic rings. If you're playing the games on a Game Boy Color, this shop remains closed throughout the adventure. If you play on a Game Boy Advance, then the secret Advance Shop opens for business and offers the GBA Nature Ring or GBA Time Ring for sale. It's a neat special thing for GBA owners to discover back in 2001 when these games and hardware were new. Unfortunately, if you play the Oracle games on a Nintendo 3DS (which, we can all agree, is much more advanced than a GBA), the Advance Shop remains closed since the 3DS is emulating the GBC, not the GBA. No GBA rings for you, weary adventurer! Thankfully, it's been discovered that you can manipulate the game's password which tracks your cross-game inventory to give yourself the two rings when moving your progress from one Oracle game to the other for a Linked Game session. Sound complicated? It's not so bad. Kolma at GoNintendo sums up the process.
Once you have beaten one of the games, either Ages or Seasons and you transfer your password from one game to the other you can visit the Ring Appraiser and talk to his Red snake to get the password from the first game and transfer it into the second. This will allow the transferring of collected rings. Using this chart you can find out the exact characters to edit in order to not only transfer your current ring collection, but also to transfer the GBA Nature Ring, and GBA Time Ring from Oracle of Seasons and Oracle of Ages respectfully.
It's not perfect, but it can be done (and this also works if you're playing the games on an actual GBC). It just takes some time and a little effort. Of course, the GBA rings don't actually do anything in-game other than add to your inventory, but you can't achieve 100% completion without them. I plan to add the rings to my inventory through this method when the time comes (I've only just begun Oracle of Seasons) even though there's no real benefit to doing so. The way I figure it, I've spent enough money on Nintendo stuff over the years that I am entitled to this old retro bonus meant to reward those who spent a lot of money on Nintendo stuff. I'm not normally one to be part of the entitlement culture, but in this instance I can make an exception.