HP LASERJET 3600 - Manual (Page 11)

NOTE: If this procedure has been properly followed, in the back of the
printer, you will see the 4 white cams that control the movement of
developers (see figure below). On the top and bottom shafts, you can see a
black plastic part that has a flag associated with it. The top flag is for the
black cartridge, the bottom flag is for all the color cartridges. Also, note that
the white cams are all at different degrees of rotation. This reduces the force
required to drive the motors when turning the cams. Metal shafts should be
oriented with the empty space downwards.