Neither Van Hasbroeck nor Lager mention this. However, the first IIIg-cameras had slightly different winding knobs, all knurled, not partially smooth.
The IIIg had the following speeds: 1/1000, 1/500, 1/250, 1/125 etc, while the IIIf had the old speeds: 1/1000, 1/500, 1/200, 1/100 etc, so I think the story is a myth.
Erik.