SSPhone, thank you for the explanation of the model numbers. It appears that I have a 128/S60, because the MPD card has an orange tab that is labeled "MPD" only. (Also, the power supply says "For use on S60 and S818 systems only," so it is an S60.)

Now I understand clearly what is required to add the four extensions. Although I will not be doing it immediately, when when we are ready to do it, I know I can contact one of the helpful experts here for the needed software and tech support. Thanks again!