I believe the 451 is the WE version of ITTs 403. Arthur is much more up to speed on WECO stuff, he'll hopefully chime in soon.
Your 551 has 18 pin slots. Again these units do not provide the music source, only the isolation. Any source, including a transistor radio will work. Clip the bud off a earphone jack and hook that up.
If you only provide music to line 1 you can skip the isolation card.
Sam
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