IBM RT PC Options Installation page 119

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