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Is there anyone familiar with the 3c blue green soft porous paper special printing? I would also like to know how to tell the difference between the 1879 1c dark ultra and the 1880 Special printing dark ultra?
If the discrimination of these paper types were straightforward and simple someone would have given you an answer on that already. In point of fact, direct comparison with known examples is the most certain way to make a final determination. The Philatelic Foundation maintains a reference collection precisely for that purpose, and a team of experts qualified to make the comparisons. If you have reason to believe that you may have a special printing, you should submit your stamp for a certificate. The charge for a negative determination is about $20-25, but if you hit the jackpot on one of these, the fee will cost you considerably more.
However, in the case of the Bank Note Issues, be advised that you can pretty much rule out used stamps from consideration.