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Shortage of revenue stamps or did the bank not know any better?
Most likely the former, although the signatory is the same on all of the checks so it is possible that they didn't know any better.
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Is it unusual that postage stamps would be used as revenues during this period?
Using U.S. postage stamps for revenue purposes was illegal, unlike in many other countries. They are uncommon, but not rare. A common cover with either of these stamps would be a buck or two, whereas these particular documents are each $20-40 on average.
Older documents from the Civil War era with illegal usages can vary greatly in value, from $20-50 for a common 2-cent BlackJack or 3-cent George Washington used as revenue, all the way up into the $300-500 range, depending on the document and the stamp (or stamps) used. Things like the 1869 pictorials, grilled stamps, combination usages, etc. can make for scarce and more expensive documents.
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Also why would "pay to the order of" be crossed out in several examples?
That one I can't answer.