MIB: ENTITY-MIB
The vendor-specific serial number string for the physical
entity.  The preferred value is the serial number string
actually printed on the component itself (if present).
On the first instantiation of an physical entity, the value
of entPhysicalSerialNum associated with that entity is set
to the correct vendor-assigned serial number, if this
information is available to the agent.  If a serial number
is unknown or non-existent, the entPhysicalSerialNum will be
set to a zero-length string instead.