IBM RT PC Options Installation page 134

To install the IBM Personal Computer AT Math Co-Processor,
align the pins on the module with the connector on the IBM RT
PC Personal Computer AT Coprocessor Option. Align the notch on
the module with the notch on the connector. Press the module into
place.
Warning: You must install the IBM RT PC Personal Computer
AT Coprocessor Option in adapter slot 8 in IBM 6150 and in
adapter slot 5 in IBM 6151.
Installing: See the system unit diagram for the correct adapter
slot.
4-6 Options Installation