Compaq Armada M300 Series Reference Manual page 66

The mobile expansion unit is available on select models and can
be purchased as an upgrade for other models.
In addition to the mobile expansion unit, the computer can also be
used with other compatible Compaq docking devices. The
computer must be docked in the mobile expansion unit before
docking the entire system to a docking device. For more
information on the individual docking solutions, contact your
local authorized Compaq reseller or provider.