The notes had 150 minor marks that acted as security measures to identify forgeries. These were often assumed to be printing errors, and were changed between issues of notes. Each note bore an alphanumeric serial designation and the signature of the Chief Cashier of the Bank of England. Prior to the release of any notes by the Bank of England, all serial numbers were recorded in ledgers so the bank could verify its liabilities; these numbers were checked when the notes circulated back through the bank.