IBM 3151 User Manual page 63

Defining Function Keys (IBM Mode)
definition at this point but do not want to save the definition, press the
Del
F
key while holding down the Ctrl key.
If
you have changed the definition and
press the
Send key, only 64 characters will be stored in the display station.
0
~
2-38
User's Guide for the Cartridge to Emulate IBM and DEC Terminals