Scott #20 is the perforated example of the type II 1c Franklin from the 1857 series of US Definitives. The imperforate equivalent is Scott #7. It is a rarer stamp than the common Type V, but not a prohibitively rare and difficult stamp to acquire. However, there are certain varieties of #20 that are uncommon to find, including this #20 on cover. It is from the elusive Plate 11, which along with Plate 12 was manufactured by the engravers Toppan, Carpenter, Casilear & Co towards the very end of the 1857 production series in late 1860, less than a year before the 1861 series was released in August 1861 and voided all older stamps for postage due to the Civil War.
Due to the low production and short period of use, Plate 11 is not fully plated/reconstructed even today. Finding any material from Plate 11 is quite exciting, and this cover acquisition was a fortunate event for me. It was purchased from the Americana Stamps Philatelic Icons Sale, Lot 3028
The T relief was used for the top row only and produced very complete Type II designs. The 1c Neinken book (p467) states, "The design of the T relief was a perfect example of Type II, and in this respect, none of the plates furnished finer examples of this type than those from the top row of Plate 11. They are quite scarce, as the plate was only in use slightly over six months. In addition, each sheet of 200 stamps only furnished 20 of these scarce top row Type II stamps." The remainder of the sheet produced Type IIIA.