That was my first thought but the spacing between the S & G appears to me to be wider than the normal 'SG' stamp and right for the S'G' stamp. The S'G' mark on this housing appears to be as originally struck - whether put on by the factory at the time of manufacture or by others after the fact. I don't recall seeing any S'G' metal stamps being sold in the past few years so I am inclined to believe this is a factory original mark - even though the books say it can't be. Also I can't believe it would be worth the time and cost to scrub a light NPM mark and fake stamp the S'G' mark. It just doesn't make sense to me.
Perhaps we will get some additional commentary after the auction closes. I hope so. ChipS