I must have too much time on my hands, because I actually looked this up in the handbook on handbooks that aeroalias referenced.
18.104.22.168 Document identifiers
The identifier for handbooks shall be placed in the upper right corner
of the first page. The identifier as assigned by ATO-W Enterprise CM
shall include the letters “FAA” followed by a hyphen, then any
combination of numbers, letters, or dashes. It shall not include any
22.214.171.124 Handbook numbering.
FAA handbooks are assigned traceability numbers by Enterprise
Configuration Management organization following endorsement by the
Systems Engineering organization. This alphanumeric designator will
utilize the following format: FAA- HDBK-(XXX), where "XXX" equals a
three digit number. The handbook number is placed in the uppermost
right corner of the cover page.
Section 126.96.36.199.1 Superseding revisions doesn’t appear to be followed in any of the FAA handbooks I have.