w/o digging a book out....
IIRC the rkp & expander where on the same data bus & need a 5th wire for rkp sounds ?
Abacus used to use funny resistor values & could have spilt zones (a&b) via resistor vaules or "peardrops"
There was an 6/14 with a knacked rkp + expander kicking around my unit floor the other month