IBM 3151 User Manual page 139

Reference Information (SYSTEM ATTACH)
pa2:
Bit Content Description
7 B'O' Always 0
6 B'l' Always 1
5 B'O' Not used
4 B'O'
Not used
3 B'O'
Not used
2-1 Characters
B'OO' National-
B'OI'
Reserved
B'IO'
Reserved
B'll'
All
means the default setting.
5-30
User's Guide for the Cartridge to Emulate IBM and DEC Terminals