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Scott ForrestTed could very well be right. Perhaps a bunch of early sheets were destroyed because of print flaws and they figured while they were fixing those images they would get rid of Plank and Wagner as well, Plank for unknown reasons. Then Plank reappeared briefly in another series and disappeared again for some unrelated and also unknown reason.
It also is weird that there are no stories about printing these cards, especailly from the early years of the hobby when those involved would still have been alive.