IBM RT PC Options Installation page 180

IBM RT PC Extended Monochrome Graphics Display Adapter
You can install one IBM Advanced Color Graphics Display Adapter in a
system unit.
Installing: You can install this adapter in adapter slot 2, 4, 5, or 7 in IBM
6150 and in slot 2, 3, or 4, in IBM 6151, whichever is available. See the
system unit diagram for the correct adapter slot.
Section 7. Display Adapter Options
7 -1 S