The Power Wah Boost is pretty extreme. I think it explains the tone in the video at the start of this thread. Check out the demo:
At extremes the wah basically filters out so much that you hear next to no signal passing through. I think Cliff was using the Ric-O-Sound output on his bass, with the neck pickup driving an OD or fuzz through the Morley Power Wah Boost. His bridge pickup is running through a limiter (MXR or early Boss?) into a seperate amp. We are hearing the bridge pickup (maybe just a Jazz pickup, or with the Strat pickup in the bridge cavity as well), all the the time. The Morley is so extreme as to be basically mixing the dirt in and out of the signal, as well as that wild wah tone.
I like Cliff's slightly exasperated little hand gesture when he wants Lars to come in on the drums. It is almost like he wanted Lars to be more of a musician and to be following his little bass suite, but instead it was back to rehearsal room visual cues.
Who knows!