and I would presume that some sort of boroscope could easily give a magnified view. Looking at photos of the 2nd failed shaft, the corrosion was pretty obvious once it had been cleaned up.
I think people are presuming too much.
So, I am back to where I started. Namely, the Bond failed shaft was the only one with corrosion at the plug shamfer, and it failed. The MGBs with the bad shamfer (known S/Ns) at the plug have been removed and re machined. Accepted.
The CHC gearbox was the only one which had corrosion under the sludge and it also failed. Other aircraft operating in the same environments have had sludge shown on NDI and cleaning, but have no corrosion, I am not sure about that.