HP A3550A - High Availability Disk Arrays Model 20 Storage Enclosure User Manual page 344

Connector
CAUTION Do not force memory module into connector or against the
Figure 7-13 SIMM Installation
7-20 SIMM Removal and Replacement
A. Position memory module above
connector so its notch is over
connector end nearest to the SP
board's edge connector.
B. Insert module in connector so it angles
Memory
slightly toward the center of SP board.
module
C. Gently push top of module down
until it snaps into each locking tab.
locking tabs. The notch at the end of the module prevents
incorrect installation.