IBM 6643 User Manual page 62

Note: If you are replacing an existing DIMM, remove the existing DIMM.
6. Position the new DIMM over the socket, making sure that the notches in
the DIMM are aligned with the tabs on the socket. Push the DIMM straight
down into the connector until the retaining clips close.
7. Replace the system board cover.
8. Replace the hard disk drive.
9. Replace the rear cover.
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User Guide
Notches