IBM RT PC Options Installation page 121

10. Adapters in system board slots B, C, and D have no screws or
brackets. Slide these adapters into the slots as in Step 7.
Adapters in system board slots B, C, and D are retained by:
• In IBM 6150, the left cover holds these adapters in place.
• In IBM 6151, the plastic option retainers hold the adapters in
place. Open the retainers, slide the adapters into the slots as
in Step 7, and close the retainers.
Section 3. Installing Adapters in All Models 3-13