Alloy differences and heat treat is what I understand. The first Wby Mk V I had was straight from Roy Weatherby himself in the 50's. I had it barreled to a 270 Win. (yes it was a standard bolt face) After bluing the receiver was a real pretty plum. If I remember someone there said the nickel content did it.

Also there is a plum spot on my Anschutz 1413 at the harder lug lock area.