It's a crazy hobby in some ways.
Sometimes something was printed using a different process, or even using a different press. And that can make a huge difference in value. As can sometimes slight differences in the paper used.
Cards are much easier!
Here's what's probably the best "find" I'll ever have.
A very common stamp, something like 65 million issued. Except, Plate 40 which can just be seen at the top was only used on an experimental steam powered press along with plate 36. Only about 2400 impressions total (240,000 total stamps from each plate) . I wrote an article about finding it for a stamp publication in 2012, the last time stamps from that plate had even been described was 1932. There are a few probable copies now that what the details were, but this is still the only certain copy from either plate. It would be a maybe if they'd gotten the centering right... The print quality is not great, about half of the 2c were done on the same press, and the print quality on those is often very bad. So bad the government insisted that steam powered presses not be used for any production, a contractual ban that lasted from 1873 into the late 1880's.
