I have never operated on the 727 but on the classic 747 models that were fitted with reserve fuel tanks 2 & 3, the ASIs had a NORMAL/AUX FUEL switch in the right hand bottom corner of the instrument. With fuel in the 2 & 3 reserve tanks (which had to be full if used) The ASIs were selected to the AUX Fuel position, which reduced the VMO pointer and VMO warning to a lower speed. I wonder if the discrepancies of VMO on the 727 had a similar origin of fuel tank configuration between certain models?