This series supplements the main series of accession records maintained by the Office of the Registrar; it contains fifty- two (52) letterpress volumes of outgoing correspondence, numbered in sequence with the prefix "A", consisting mostly of acknowledgements to donors for specimens sent to the National Museum. The correspondence is arranged chronologically. Beginning in June 1885, accession numbers are usually given on each acknowledgement, including accession numbers 15550 through 147007; accession numbers do not run in strict sequence through the volumes.

The acknowledgements contain a brief description of specimens. Each volume contains an index to personal names occurring in the correspondence. A volume list follows.