It means all Kadabras that were specifically caught as a Kadabra, as opposed to being evolved from Abra. The item byte in G2 piggybacked off a byte in the G1 data structure that was reserved for catch rate, even though it was never meaningfully used on the player's Pokemon there. Abra has a different, higher catch rate than Kadabra, but once you catch something in RBY, that byte is set to the catch rate that it had in the form you encountered it, and does not update if it evolves to a different species with a different catch rate.
The developers knew about this plan for the future of the catch rate byte in time for Yellow development, because when you get the starter Pikachu there, the game runs a routine to change its catch rate so that instead of using the normal catch rate for Pikachu (which is indistinguishable from the value on things like Growlithe and Psyduck), it sets a signature value which would go on to be reserved for the Light Ball.