I wish I could find the pic but I can't. When the flexplate came off it came off in two pieces. Just the little flange was left still bolted to the crank.
Theory is, once it begun spinning, centrifugal force held it together tight enough to silence the knock, and the heat once the engine warmed up expanded the parts enough to also keep it from knocking. Once it cooled off, it would knock again. I can't believe it held together as it was but I suppose the torque converter wouldn't let it walk back and separate.
This is a pic snagged from the webs but mine looked essentially like this:
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(pic credit: "Hitcher" on 67-72 trucks forum)
LOL.. I "kinda" found a pic of my old flexplate. Here it is in a roll-off dumpster at my old place in Atlanta. You can't see the break in it, but that's why it's in there..
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Richard