Apple PowerBook Duo Dock (including Duo Dock Plus) - Manual (Page 39)

8 Turn the subassembly over and put it down with the disk drive(s) facing up.
9 Push out the plastic cover plate behind one of the NuBus slots and set the plate aside.
Use your thumb to push out the cover plate.
About NuBus slots and cards: The two NuBus slots in the Duo Dock are
stacked vertically. The slots are identical, so you can install your card in either
one. If you install your card in the upper slot (which is the lower slot when
the dock is right side up), you’ll have to remove it and reinstall it if you want
to add a second card later.
NuBus cards come in different shapes and sizes. All properly designed cards
will fit into the Duo Dock. You may find it easier to install a large card in the
upper slot rather than the lower one.
Hard disk drive (optional) Floppy disk drive
Push out either cover plate.
33 Installing a NuBus Card