Yes, but what was the number of forgeries found? Mint ones do not exist, apparently.
The telegraph forms that these were used on were stored for years before they were supposed to be destroyed eventually; most were destroyed. The monograph mentions a few "leaking out" (not part of the Nissen discovery group) and then "large leak" of telegram forms in 1898 and one in 1910, whatever those numbers could be. This means all stored forms and does not mean that all the forms had forgeries. It also took over 20 years before Nissen noticed that forgeries existed and it can be assumed that a lot of brokerage house records with telegraph forms were already destroyed.
One of the great unsolved stamp mysteries of all time. The forger got away with it. There was an article recently n a British stamp magazine and the author speculated it was an inside job with the forger supplying the stamps and a telegraph clerk affixing them to forms, pocketing the money.