MIB: ENTITY-MIB
The vendor-specific hardware revision string for the
physical entity.  The preferred value is the hardware
revision identifier actually printed on the component itself
(if present).
Note that if revision information is stored internally in a
non-printable (e.g., binary) format, then the agent must
convert such information to a printable format, in an
implementation-specific manner.
If no specific hardware revision string is associated with
the physical component, or if this information is unknown to
the agent, then this object will contain a zero-length
string.