Ever since revision F1, the 1A2 seriss boards could be linked
together with a ribbon cable so as to manage 4 CO lines and 8
separate extensions that can each be dialed on the intercom line
by dialing "1" thru "8".
The general docs for interlinking cards can be seen in the
"Expansion" chapter of the User
Manual, which in the case of that REV-J5 manual, it starts
on page 19. Or, this single page diagram showing the simple
connection notes:
http://seriss.com/1a2-ksu/rev-j5/data/1a2-REV-J5-0015.png
Some actual hook up notes: I suggest mounting both boards to
plywood back board about 2" apart on rubber spacers, horizontally
aligned, like this:
This way the ribbon cable can be run underneath the boards, so
it's out of the way - folded "under" the boards with bends (but no
"twists"):
The new REV-J boards also have slots to allow one to utilize velcro straps (optional) to retain the connectors.