Posting for a buddy:

The HP 8640B comes standard with a 400/1000 Hz AM modulator. The entire
assembly, including front panel knob, is mounted on a plugin board.

The Option 01 replaces the fixed frequency modulator with a variable
frequency unit. It, too is mounted completely on one plugin board, including
front panel control.

When he attempts to swap the 01 variable frequency modulator into a unit
which was equipped with the fixed frequency modulator, it doesn't work, and
the FM modulation, which is located in another part of the instrument, fails
as well.

These results are constant over multiple units.

Why?