Congratulations! I was having a look at the photos of that rifle and the serial numbers. A bit of a puzzle, although I am no expert by any means...
It looks to me as tough someone may have started to buff the finish off the left side of the wall - possibly for re-blueing? - but then thought better of it. Can't think why. You're right, something definitely doesn't smell right about the bolt shroud, but maybe the same thing happened there? I don't know, but as you say, I don't think it would bother me... I could see no evidence of a side-mount for a scope in the photos.
I was looking at the Rigby and Mauser serial numbers in Speed's 'original oberndorf' book. It looks like the mauser action is a short-intermediate but following the regular mauser number series, not the special rigby series for these actions. Nothing unusual about that - there are other examples in the book like that. This rifle is not listed in the book - the seller says c.1907 but the Speed serial numbers would suggest 1909. The Rigby serial numbers would suggest that the action was completed by them between May 1909 and August 1912 (this is the date gap in Speeds book). My guess is 1910, possibly 1911. What I find strange is that the rifle is not listed in either the Mauser serial number list (not uncommon) but neither is it in the Rigby S/N list - being a take-down I would have thought that it would have stood out (whereas WR, Jeffery and H&H take-downs do in the book).
A nice find and definitely worth looking at closely! Am interested to hear your thoughts/comments