Man, Paul! You definitely have this stuff under control. I'll do my best to answer with what I know:

As for your first question about the 4200A card, I really don't know for sure, but I'll take a guess. I don't have any of their schematics left to review. Bridged ringing and grounded ringing were options when party lines were still around. They were definitely present when 1A2 was popular. I am assuming that this option was to select between the tip side of the line or ground for ringing voltage reference. Again, that's a guess.

As for question #2 about the manual intercom: Yes, this was just a common open talk path that appeared on a line key through traditional 3-pair cross-connects. These were generally set up as private talk paths between an executive and a secretary, but anybody with an appearance of this "line" could barge in. Very frequently, the locking screw of an unused line key could be removed to become the pushbutton to buzz the other station. A momentary ground (A1) was simply placed upon the line key's A lead while pressed which closed the circuit to the buzzer at the other station.

In cases where multiple stations were involved, "side cars" of buttons were added to the phones and spare leads in the line cord were used.

Yes, there was a dial intercom card made by ITT and San/Bar that fit the standard 1A2 "sized" card slot. This card, while similar in size, could not be installed in regular slots; it had to be a dedicated slot. Western Electric had a 424C and later 494A card for dial intercom, but it was an 8" card as opposed to the traditional 4".

Yes, the ITT and San/Bar cards had option jumpers to select MOH. Their factory setting was no MOH, so one would have to change them and hopefully during this process, think about checking for the presence of A battery.

Ed Vaughn, MBSWWYPBX