The 1851 1¢ Blue Benjamin Franklin stamp, specifically the Type Ic (Scott #6b), is a high-value rarity in US philately defined by its unique design "trimming.

It originates exclusively from Plate 4. This plate was the last used for imperforate stamps and was created in early 1857. There are only 8 specific plate positions for this type. Two of these are on the bottom row (positions 91L4 and 96R4), and the remaining are found in the body of the plate from "E" relief positions.

Interesting points

1) Although recognized by experts like Stanley B. Ashbrook as early as 1938, Type Ic was not officially listed in the Scott Specialized Catalogue until 1993.
2) These stamps were printed only during April, May, and June 1857. By August 1, 1857, imperforate stamps were replaced by perforated versions.
3) Out of a survey of over 2,500 off-cover 1-cent stamps, only 56 were found to be Type Ic.
4) They were only printed in the months of April, May and June 1857, a late period in the printing of the imperforate.
5) The imperforate stamps were replaced with the perforated stamps on August 1st 1857.
6) A certificate is a must, never buy without one.

How to read plate positions

The first indicator is a number indicates its position on the plate, so 3RIE would have come from the third stamp on the plate. The number can range from 1 to 100, there being 100 stamps on each plate.

The second indicator is either the letter R or L, R indicates the stamp came from the right pane, L for the left pane. The stamp was printed in sheets of 200, each sheet was further divided into two panes of 100. Hence 3RIE would have come from the right pane as the second indicator in 3R1E is the letter R.

The third indicator can be from numbers I (1) to XII (12). This indicator is always shown in roman numerals. There were twelve plates, there are no stamps from plate VI (6) as it was destroyed before printing began (no doubt it was flawed). For example stamp 4RIL would have come from plate one (1).

The last or fourth indicator is either the letter E or L. The letter 'E' indicate an early state of the plate, the letter 'L' indicates the late state of the plate. This indicator only applies to PLATE I, as it is the only one that has an early and late plate. The early plate is the original plate. After 11 months the plate became worn and 199 of the 200 positions were recut. 113 positions on the plate had both top and bottom lines recut, 40 positions had only the top line recut, 8 positions had only the bottom line recut, 11 positions had a double recut at the bottom and 4 positions had a double recut at the top.

How many plates were there?

There were twelve plates of the 1¢ Franklin made, plate six was never used, probably due to it being damaged in it's creation. Most of the plates were used for both the imperforate and perforated design. Some only produced one type or the other. For instance, plate 12 produced only perforated stamps and the early state of Plate 1 produced only imperforate stamps whilst plate I late (reconstruction) produced both imperforate and perforated stamps. Plate 4 was the last of the imperforate plates to be used.