Base structure Pokédex marker byte
The base structure Pokédex marker byte is a byte in Pokémon Red, Blue, and Yellow and Pokémon Gold, Silver and Crystal found in the ROM at the beginning of a Pokémon's "base stats data structure" (shared attributes for Pokémon of the same family) that is intended to be equal to the Pokémon's Pokédex number.
The Pokédex marker byte may serve as minor detail about a glitch Pokémon's family as it may not be equal to the glitch Pokémon's Pokédex number.
This byte according to Stag019 is supposed to be the same as the Pokémon's Pokédex number, but for many glitch Pokémon it is different. 'M (00) and MissingNo. in Red/Blue are exceptions. They have a Pokédex marker byte of 0x00, which is the same as their Pokédex number.
The location of a glitch Pokémon family's base stats data can be found using the following:
0x0383DE + (PkmnNo. − 1) × 0x1C
Presumably hybrid glitch Pokémon will have the same Pokédex marker byte as their Pokédex number, due to having their base data derived from real Pokémon.
In Generation II, it is used to determine the Pokédex entry bank.
List of families and Pokédex marker bytes
|Pokémon's Internal ID||ROM offset||Marker byte|
|Pokémon's Internal ID||ROM offset||Pokédex Marker|