The penny black stamp and two penny blue stamps were introduced as the cornerstone of Rowland Hill's postal reform of 1840. The 2d value was printed from 2 plates and the 1d value from 11.
Stamps from this period are generally relatively easy to plate in comparison with the later issues. The SG specialised catalogue (Ref 1) explains the differences between the 2d plates 1 and 2 quite well and should be a sufficient guide. The identification of black plates is more difficult, but, in addition to the position of the corner or check letters in the lower squares, deterioration of the roller dies used to put down the impressions led to small variations in the design which can be very helpful. These include, variations in the rays in the northwest and northeast stars and the 'O' flaw (a small break in the lower margin to the right of the 'O' of 'ONE').
Once again, the SG catalogue provides a good background to these and more details can be found in more advanced texts such as Lichfield (Ref 2) and Proud (Ref 3).
To add to the interest many stamp varieties, re-entries, double letters and multiple states exist. For example, every impression on Plate 1 was repaired and so exists in two states known as Plates 1a and 1b. Some impressions were repaired again later in the life of the plate and so multiple states exist. The re-entry process must have been a very difficult operation to perform and it is remarkable that in most cases the re-entry was coincident, thus there is no doubling of the design. However, in a few cases the re-entry was not coincident and so some re-entry marks can be seen.
Some impressions on many of the later plates were also repaired by re-entry and so multiple states exist. In many cases a roller die with no corner stars was used and so the stars and check letters become weaker with each re-entry.
The rarest plate is Plate 11. This was never intended to be issued in black but some sheets were printed in black as stocks of stamps grew low and more stamps were needed before the change of colour to brown.