The US Department of the Navy (which is NOT the United States Navy) provides a governmental organisational structure to oversee the Navy.
By 1938 it was structured to include a number of "Bureaus" including the Bureau of Navigation (Bur. Nav.) which had responsibility for Navy personnel, the Hydrographic Office and the Naval Observatory. In 1942 the Bureau of Navigation became the Bureau of Naval Personnel (BuPers) and the other functions were transferred to the Office of the Chief of Naval Operations.
References:
Records of the Bureau of Naval Personnel
Wikipedia)