IBM RT User's Setup Manual page 303

6-16 Options Installation
You can install this adapter in slots 2 through 7 in
Installing:
IBM 6150 and in slots 1 through 4 in IBM 6151, whichever is
available. See the system unit diagram for the correct adapter slot.
In IBM 6151, you can more easily connect cables if you install the
adapter in a slot other than adapter slot 1.