Early Eyeball Frog

Note: All addresses are NTSC 1.0

There's a byte located at 0x0683 into Link's instance that seems to be used to communicate what tradeable item an actor is currently accepting/communicates what item Link is showing to someone/nobody. Id values aren't the same as item ids, but do seem to follow a similar order at least. Link forces this value to 0 every frame if not holding an item (probably). Not entirely sure how the game uses it to communicate data since debugging is a pain in the ass.

In the case of King Zora:

Whenever King Zora can be spoken to (Speak on A), the game branches to one of two functions (Child/Adult) every frame that test whether King Zora is able to accept items. As an Adult, the game performs a rather sketchy check: It tests if the Adult Trade slot contains an item ID that is less than the Eyeball Frog Id, rather than testing specifically for the Prescription.

When King Zora awards an item to Adult Link, he's coded to check if the byte at Link's Instance + 0x0683 is storing 0x0C, which represents that Link is holding the Prescription. If the check fails, Link is awarded the Zora Tunic, whereas if the check passes Link receives the Eyeball Frog.

When performing the Early Eyeball Frog glitch successfully, King Zora actually initially sets Link's "Get Item" variable (Link's Instance + 0x0424) to 0x2D, which gives the Zora Tunic on the first frame of the trick. Delaying item collection causes King Zora to continuously try to give Link an item every frame until the actor is unloaded or succeeds. Furthermore, it also causes Link to enter Speak on A "range" again, which overwrites the Link's Instance + 0x0683 variable with 0x0C. Thus, if Link remains in Speak range, he will obtain the Eyeball Frog when the collection delay ends.

Getting the Zora Tunic instead of the Eyeball Frog is achieved in a similar fashion. Z-Targeting King Zora allows you to extend the Speak range beyond it's normal range. By Z-Targeting King Zora and backing down the steps before showing the Eyeball Frog, you end up standing outside of Speak range once the collection delay is active and Z-Target is broken, causing Link's code to zero out the Link's Instance + 0x0683 variable.