HP 3550 - Manual (Page 23)

Back and side view
1 upper rear door
2 parallel connection (available on the HP Color LaserJet 3700 series printer)
3 lower rear door (rear output bin)
4 USB connection
5 power connection
6 Tray 3 (optional)
7 on/off switch
8 EIO connection (available on the HP Color LaserJet 3700 series printer)
9 DIMM access door (available on the HP Color LaserJet 3700 series printer)
ENWW Walk around 11