NEC Express5800/320Fd-LR User Manual page 57

If it becomes rocked and cannot insert any further, push and
keep the rock lever to the left as push the device further inside.
7.
Close the lever on the top and the bottom
of the device. Fix the device to the bracket
with clinchers.
8.
Take steps 6 to 8 to fix the other device to the bracket.
Installing Server 3-17
Lock lever