After the Penny Black plates were put to press they began to wear.  Some plates, such as Plate 1, due to not being hardened wore rapidly and needed a great deal of repair.  Other plates held up well and remained in service for the entire length of time that the Black stamps were issued, such as Plate 1b.  Naturally,  since Plate 1b held up so well and did a long term of service it is also the most common of the plates.  I have found 2 examples of the Penny Black with inverted watermark and was not surprised when I plated them that they both were from Plate 1b.

When the plates reached a point where repair was necessary it was accomplished in two ways:  Re-entry and hand re-touching.  The frame lines and the check letter squares were re-cut or re-touched by hand. This was not always accurately done, and the result was rough or uneven or  skewed lines which sometimes passed beyond the top, bottom
or side and created "extended lines."  The other process, re-entry, was executed in order to eliminate wear which
occurred within the stamp design itself.  Re-entry was accomplished by rolling the die back over the previously
impressed design, redefining the portrait and surrounding details.   The die used for re-entry did not have check letters,
as these were originally hand punched into the check letter boxes.  Instead,  the die had blank squares.  When the re-entry was carried out the blank squares came into contact with the already existing check letters on the plates and had a tendency to flatten them out.  This provides the Plater with a clue that they are looking at a second state.  The stamp design may appear fresh and detailed as a result of the re-entry, but the check letters are faded and have a worn appearance.

As an aid to the original entry of the plate impressions, the roller was guided by markings that were put on the fresh plate in advance.  these are called plate dots and guide lines.  During the early stages of stamp production the roller was guided over the plate manually.   For this reason the roller did not always come into contact in neat rows where it was intended.  The stamp impressions ended up slightly along side the plate dot and guideline markings.  These markings, which would have otherwise have been covered or hidden within the stamp design, can be seen along side the top or bottom and sides of the stamp design.  In the operation of re-entry,  even greater skill was required in order to achieve a coincidental re-entry which would perfectly match the original impression.  Otherwise, the re-entry, if shifted even slightly, would create a doubling up of the design.  As a result of imperfect re-entry, the design may be found slightly duplicated,  being particularly noticeable in the check letter boxes, the star corners and sometimes the top or bottom frame lines.   This evidence alerts the Plater to the fact that the stamp under study has been repaired and is in a second and less likely third state.

Its believed that all of the Penny Black plates underwent repair, with the possible exception of Plate  3, 4, and 7.   Plate 5 is known to have undergone 3 stages of repairs.  Since a number of the Penny Black Plates were put to press with red ink near the end of their existence, the necessary repairs took place just before or during the red ink period of their use in 1841.  The fact that the plates wore and required repair, and the repair altered the impressions making them more and more unique from the original die impression adds another dimension of clues to help identify the plates.  The re-cutting, re-touching and even the re-entry were done manually and as a result were imperfect in execution. These imperfections are "blueprints" that can be used to identify the these plates.