Separate books and computer databases may be kept for the recording and indexing of different classes of instruments. Three distinct series of document numbers may be used for recording documents received for recordation, one series of numbers to be preceded by the letter “b” in each case, which series shall be used only for bills of sale of personal property, chattel mortgages and releases, extensions and assignments, thereof, one series of numbers to be preceded by the letter “c” in each case, which series shall be used only for certificates of discharge of discharged members of the military, aviation and naval forces of the United States, and the other series of document numbers shall be used for all other instruments received for recordation. When three series of document numbers are thus used, a separate place may be provided in the Recorder’s office for the receipt of each kind of documents to which such serial numbers apply.