On a related note about the 3¢ Banknote cover just mentioned.
Sometimes there are other attributes about the cover that can help you. For example, #158 was printed in 1873 by the Continental Banknote Company. Scott #184 was printed in 1879 by the National Banknote Company. If this cover had a postmark with a year date of, say, 1875 – then this cover could not possibly be #184. Alas, the cover my customer sent to me had no year date in the postmark. Rats!
Before you go removing stamps from covers for identification, sometimes there may be other characteristics about the cover, such as the postmark, that can aid you in identification.
Be careful though, because sometimes you can be fooled. In this case, suppose there was a letter inside the envelope to Aunt Minnie. The letter is dated July 15, 1875. So that makes this cover #158, right? That is a strong maybe, but not a guarantee. Unless there was something unusual about the letter, it’s impossible to tell if the letter originated with the cover. What if someone read the letter and put it back into the wrong envelope?
Now, if the writer said, “Hi Aunt Minnie. I’m visiting Clara in Buffalo, NY and we’re going to the 1901 Pan Pacific exposition …” and the cover was postmarked in Buffalo and the envelope had a printed corner card for the Pan Pacific exposition, then yes, it’s almost a sure bet that this cover originated in that envelope. This is going to be the exception that something in the letter helps positively tie it to the cover.
Another possibility is that the cover has a postmark from a post office that closed. Suppose that post office closed in 1875. Again, that means it is not Scott #184 assuming the postmark looks legitimate.
Sometimes the covers can speak volumes to us. Learn to be observant and listen.
I hope this has been insightful. I hope you learned something new today!