Evidence of the Stock Exchange Forgery surfaced first in early 1898, when Charles Nissen, the author of The Plating of the Penny Black Stamps of Great Britain, 1840, discovered examples of "Plate 5" and "Plate 6."  The forgeries were used on telegraph forms at the Stock Exchange, and would have escaped notice had they been destroyed after use which was the normal procedure. They apparently, based upon the dates on the circular date stamps, were used during 1872 and 1873.

The telegraph forms were franked with the forged stamps and the clerk participating in the fraud would pocket the cash.  Since
the clerk used the forged stamps, the stock of genuine stamps would not be depleted, thus concealing the fraud.   The public would get nothing more than a brief glimpse at the telegraph form, which would be filed and later placed in bags for destruction.  The discovery was made possible due to the fact that the bags were taken from storage instead of
destroyed and the stamps were sold to dealers.

The forgery of Plate 6 is scarcer than Plate 5 and somewhat better executed.  The stamps from both plates exist with "normal"
check letter combinations,  and non-existent combinations.  The forgers made the error of creating check letter combinations
which exceeded the normal range of the letter "L."   The forged stamps are similar to the issue of the period, with large white
check letters.  The physical characteristics of the forgeries are as follows:
 

Detail of north-west corner. Note the blunt corner. On the genuine stamps the corners of the stamp and other details are much sharper.