I suppose Im posting this just to share my experience that may be of some use.
Thread on E420 hub were stripped on my previous E420 and now I have stripped the ones on my current E420. LAst time it was rear, this time front. I have owned around 11 or 12 w124s and this has only happened on e420s. The metal seems different, maybe softer?
Wheel hubs for w124 e320 and e420 e500 are 2 different items and the wheel bearing will not fit. I got a e320 hub thinking its the same, but the bearing race was different size by maybe 1 or 2 mm.
For the E420 front wheel hub is 129 330 04 25
This part comes in sl500 and I think Sl300 and the E500 as far as I recall.
Anyway, an additional point: Tapping or rethreading the wheel hub results in a small amount of metal removal. I tested a standard thread versus the RE-threaded and there was slightly more wobble with the Re-threaded one. So, I may install a new hub (not used) or try to see if it is practical to use seom form of threadlocker (which I am not convinced is the best idea as I'm always fixing something behind the wheels). Keeping a torque wrench in the car all the time is going to be the norm for a while untill I am assured that there is no loosening of the bolts.