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Going through my old PC and backing up files I came across a bunch of pics I have on this. The 73-75 push pull dual tank setup used a cable operated switching valve. A guy was selling this and I didn't need it but wanted it for the nostalgia, nevertheless I will add the pics I collected here for information purposes.
OK, now I'm thoroughly confused. Here are the pics I just took, of the selector switch in my '74, which came with dual tanks from the factory. It's identical in every way from what I remember from our 74 and 76 crews, and my own 76. None of the three of them ever required you to do anything other than merely flip the switch - they were definitely not momentary switches. I never even saw a tank switch with the words "Depress Fully" on the switchplate until our '85. It was definitely a momentary, because it had a short somewhere that would blow the fuse if you held the switch down too long.
As far as I can tell 73-75 had the push pull option and then they were all electronic switching valves after and for replacements. My 74 GMC Sierra Grande 454 1500 came with dual tanks and push pull setup.