OK Lu, I had the opportunity to check a 412 with a Magnaflux field indicator, which I presumed was an oersted/gauss meter.
The helicopter had not flown for app. 8hrs. The readings ranged from about .75 to 1 gauss at all points accessible on the mast from the mast nut to the top of the collective sleeve
The limits indicated in the Bell SPM were 3 gauss or less. So it does not appear to be a real problem for this particular machine.