Before 1900 an individual Certificate of Discharge would be given to a merchant seaman as he left a ship, but there was no identifying number and individual sheets were easily lost.
The discharge book for merchant seamen, or more formally the ‘Continuous Certificate of Discharge’ , was introduced in 1900, for foreign voyages only. It appears that it was introduced to try and ensure that only men of good character were engaged. Each book has a unique identifying number (Discharge A number) and includes personal details, foreign voyage details and character reports. The seaman held the book, but handed it in to the master for the duration of the voyage.. Early books had 42 voyage slots but this was increased to 60 by 1917. A replacement book was issued if the old one was full up or had been lost.
With the introduction of the Fourth Register of seamen in 1913 a CR1 central record card was completed every time a discharge book was issued.
The Discharge A numbers were originally numbers only and had risen to 1140000 (7 digits) by the mid 1920s. Numbering started again then with a letter prefix to indicate the seaman’s country of origin. The prefix for British seamen was usually R and numbers had risen to around R900000 by 1972 when the system was changed.
It appears that from around 1939 home trade voyages were included in the discharge book and the individual discharge sheets disappeared.