Western Electric type 400D KTU's of issue 15 and later will respond to bridged ringing. Earlier issues will only respond to grounded ringing. They will all work reliably on loop voltages of 20 volts and up, assuming that the line current is at least 20 mils.
As Ed says, E, F, G and H versions will work on bridged ringing. Also, SanBar 4000-series will work, too.
There is a very simple bridged ringing modification that one of our TCI members has come up with to convert WE 400D cards made prior to issue 15. It involves cutting one lead and soldering in a resistor on the daughter board.
If you're interested in seeing it, I'll forward the pdf to you privately.
