The fourth card slot can certainly be used for a manual intercom. The only thing that card will do is give you a line light and talk battery. There's plenty of spare pairs to use for manual buzzers. A better alternative is a Valcom or Teltone dial intercom. These are separately mounted boxes with (usually) a 25 pair connector on the side that you break out on a 66 block. You give it A, B, and Lamp battery and what ever source you choose to use for signaling. Most of the buzzers out there run on 10VAC lamp battery. Also tip and ring will come out and be multipled to all your sets. They usually don't have a leads, since it can just detect off hook and give you lamp illumination. Not too much need of putting an intercom call on hold since it stays set up even if one person hangs up. It will give you signalling outputs to connect to the buzzers in individual sets. Typically you put the buzzers on yellow/orange or yellow/green. Most sets have the hold lamp on y/g. Just cross connect everything and there you go. (HIGHLY GLOSSED OVER)

You can certainly use the buzzers for common audible ringing. Wherever you would connect the 118A ring gen, you just put lamp battery (assuming 10VAC buzzers) and then put common audible on the yellow/orange pair instead of yellow/slate. (Again, assuming you put your buzzers on y/o.)

Stick with the ITT400E line cards. They are very good and will work with almost any source of dial tone.

If I look I might have a spare intercom add on box..... just have to dig through some dusty boxes. I have a Valcom RT-18 or whatever they called the tone and rotary 18 station model, but I plan on using that. My ITT 501 ksu came with some 10 station intercom box that I think is tone only, but I have to look. If tone only is fine, PM me.

I don't think you have to do any rewiring to use a manual intercom card in slot 4. I know the 551C's had A battery on pins 3&18 for manual intercom cards. I don't remember if you had to change the wiring on a 551B or if it was set up the same... I personally like the add-on boxes because they're dial select. No need to convert pick up keys or have little side cars attached to the phones.