The Penny Red was Great Britain’s longest running stamp, from February 1841 to the end of November 1879. It was used for the standard letter postage rate of 1d and approximately 21 billion were issued. Until 1854 the 1d red was imperforate.
The Penny Reds are identified by “plates” and their position on the plate. The bottom row has two characters (top rows have these characters reversed in perforated stamps). The character in left corner represents the row of the stamp on the plate and the right corner character represents the column of the stamp on the plate. So in my stamps shown above, the first stamp was on 19th row and 10th column on the plate. Plate numbers on perforated stamps can be identified with the help of a magnifying glass. The plate number is printed in the side bar design of the stamp. See image below for an example of a stamp from plate number 148 (this is not one of my stamps).
Penny Reds can be as interesting to collect as Machins though probably harder to collect since they are not as easily available as machins.
Some useful links for collecting Penny Reds: