A 403 card was an ITT product, not WE. As I recall, you fed music in from a source (tape deck, fm radio, etc) in on pins 3&18 and looped it (unless it was already looped) to all the lines you wanted MOH on. Some line cards (Sanbar 4200 maybe?) did not need a 403. Others did. Pins 3&18 often came with A Battery wired to it (401, 415 and other cards required it). If it was there you had to remove it.
I'm really digging deep for this stuff, so forgive any errors.
Sam
