Central Index System:- Fourth Register of Seamen 1913-41
Three earlier attempts to create a central register for seamen had ended in 1857 and were not resumed until 1913. This register consists of three types of index cards held in four separate indexes.
CR1 and CR2 cards between 1913 and late1921 have been destroyed although the CR2 voyage information between September 1918 and December 1921 has been entered on to the CR10 card.
BT364 is different. It appears to have been begun in 1941 by collecting together all pre 1941 CR1,CR2 and CR10 cards for those regarded as active at the time and then added later CR1 cards until the individuals left the sea.
Central Index Register
There is more information on this website on the elements of the system from the discharge book to the various types of record card.