Outgoing letterbooks for the period to the end of 1936 bear original numbers 1-118. These show that some letterbooks are missing. Between 1936 and 1951 carbon copies of letters were generally bound monthly in soft covers. The list below has allocated reference numbers for missing volumes in this period in the expectation that these will be located. From January 1951 letters are no longer arranged strictly by month and no allowance has been made for missing letterbooks in the series after that date.