Easily 95%+ of the stamps offered, by non-students of the issue, as #10´s are in fact #11´s. The simplest way to insure that you are getting what you pay for is to purchase a PLATED copy.
A plated stamp is one that a student of the issue has determined what plate the stamp was printed from and the position of the stamp in the plate. Those that plate stamps will write the plate position lightly in pencil on the back of the stamp.
#10´s were printed ONLY from plates 1-early, 1-intermediate, 2-early, 0, and 5-early. Each plate consisted of 200 stamps, a left and right pane of 100 stamps each. A notation like 27R2E on the back of the stamp means that it came from position 27 of the right hand pane of 100 stamps from the early state of plate 2. 48L0 means position 48 left pane of plate 0.
When purchasing any stamp that requires knowledge of subtle color, type, paper, etc. differences, it is essential that you KNOW YOUR SELLER!!! Unknowledgeable sellers far too often misidentify inexpensive stamps as the much more expensive variety out of fear of selling a stamp too cheap.
That´s the short of it. For those interested in picking up a little more knowledge, additional info follows.
* * * * * * * * *
Some sellers prey on the inexperience of collectors and use the ambiguous color descriptions in catalogues to intentionally defraud while many others are just simply unknowledgeable about the 3c 1851-57 issue. Unless one has had extensive experience with #10´s it is next to impossible to separate them from #11´s.
#10 orange brown exists in several different shades. #11´s come in many distinctive colors and shades some of which can easily be confused with the #10 colors. Just because two stamps have different colors doesn´t mean that one of them is a #10.
The color of a #10 cannot be described in words, "orange brown" being meaningless and quite worthless, and there are no other stamps of a similar color to compare it to. Many sellers see the listing of #10-orange brown and #11-dull red and naturally conclude that they have an orange brown stamp. Dull red immediately brings to mind a color that is totally different from what it really is and with that in mind the majority of #11´s appear to be much more brown or orange than red.
The bottom line factor for separating #10´s from #11´s is determining what plate the stamp was printed from. It doesn't matter one whit whether the color appears to be orange, brown, red, blue, or green with yellow polka dots, by definition and regardless of the apparent color, #10´s are from plates 1-early, 1-intermediate, 2-early, 0, and 5-early. #11´s are from plates 1-late, 2-late, 3, 4, 5-late, 6, 7, and 8.
Any cover docketed or with a letter dated or folded letter dated from July 1851 through September 1851 has to be a #10. These dates can be extended through December 1851 if it can be determined that the stamp is NOT from plate 1-late.
1L stamps are usually quite easy to spot. On 198 of the 200 1L positions the recutting of the UR diamond block shows it to have been gouged out at top. All other #10 and #11 positions except one show the UR diamond block recut with a straight line at top (lightly to heavily) or not at all recut. 1L stamps on October-December covers may be in what is called experimental orange-brown.
Please note references to UL in the above photo should read UR
In the early 1850's most individuals and businesses did not purchase stamps in quantity to use at their leisure but instead just for their immediate needs. As #10's were last printed in the latter part of 1851, the chances of finding one on a cover dated after 1851, starts going down very dramatically. From at least January 1852 on, ONLY #11's were being printed and the number of #10's still available for use dwindled rapidly in comparison to the #11's. By July 1852 it was quite unusual to have a #10 still being used.
From this you can also see that any off cover copy with a January through June cancel means that you should exercise great care when buying. This doesn´t mean that later uses don´t exist or are extremely rare, but it should certainly raise your eyebrows.
There is much interesting and useful info on the internet about the 3c 1851-57s. Good starting points might be:
Just remember that while photos may look nice there is absolutely no substitute for using properly identified real stamps for color identification.
I would recommend buying any PLATED #10´s priced under $20-25 forgetting condition, with black or pen cancels, no colored cancels. These reference copies will pay for themselves many times over used as a "color reference" and will as a bonus hold their value and quite possibly increase in value even being damaged reference copies.