To Stretch goes a tip of the hat, and a bottle of cheap bubbly, for the information he has supplied in posts #9&10.

In #9 the photos are of a rotary idle air valve. The armature and the valve pieces rotate to open and close the window in the air passages.

In #10 the drawings are of an axial idle air valve. The armature and the valve piston slide lengthwise on their axis to open and close the air aperture.

The rotary valve (as used on the OP's M103) has a no-power rest position that is slightly open, but apparently not defined, leading to observations of differing no-power idle speeds.
The axial valve does have a spring loaded, defined, no-power rest position that is wide open.