IBM RT PC Options Installation page 108

Adapters in system board slots B, C, and D have no screws or brackets.
Slide these adapters into the slots as in Steps 6 and 7.
In mM 6150, the left cover holds these adapters in place.
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System Board
Slots A, B, C, and 0
In IBM 6151, the plastic option retainers hold the adapters in place. Open
the retainers, slide the adapters into the slots as in Steps 6 and 7, and close
the retainers.
Refer to the system unit diagram for the locations of the adapters. Use the
table of contents and go to the instructions for the specific adapter.
Section 3. Installing Adapters in All Models 3-13