HP Color LaserJet 8550 Multifunction Printer series - Manual (Page 210)

EN Supported DIMMs 209
5 The DIMM slots are paired: 1-2, 3-4,
5-6, and 7-8. The ROM (font) DIMMs
are placed in odd numbered slots.
Place DIMMs in the next available
slots.
6 Push the levers down as shown.
7 Match the notches on the DIMMs with
the tabs on the connectors. Firmly
push the DIMM into the slot until the
levers lock. Repeat for each DIMM.