I (Kurt) just took the pin names from the 26A1, 26A2, and 26G3 manuals and merged them here.
I'm unfamiliar with the 2600 series, so I don't know how to generalize about the purpose of the pins.
Also, Tekwiki is lacking the 26G1 and 26G2 so I don't know how those plug-ins use the 2600 interface.