Phonenut -
I only put in two of those 601s and that was about 20 years ago but as I recall they came with a printed circuit interruptor (that always burned out!) - that might be your unnamed card.
The 403 is a buffer for the MOH. It allows you to provide MOH to all lines without inducing xtalk into the system.
The prints for those 601s are floating around - I'll see if I can dig them up and let you know for sure.
Sam
