I bought a Mauser Model 66 S (Standard) in unfired new in box condition back around 1990. It came with a .30-06 barrel and also had an unfired .243 Win barrel. It was part of a 5 gun package deal I got from the estate of a retired gunsmith, and had been tied up in probate since around 1968.
These aren't very common. I think total production was only around 50,000, and I don't know how many were sold in the U.S. It appears they were only built from 1965 to 1971. The Teutonic stock design on yours looks just like mine, except mine has a rosewood forearm tip, and a skip-line checkering pattern. Hard to tell from your pics, but the checkering pattern appears to be pretty finely spaced. And yours has much nicer figured wood than most I've seen. It also appears that yours has double triggers, while mine is a single trigger. I would assume your stock is original, and may be the more valuable Diplomat Model.
The unique telescopic short action on mine is very smooth, almost like a Krag rifle. Because the action is so short, they have a total length closer to a carbine, even with the 21" barrels. The receiver ring on my .30-06 barrel, which is an extension of the interchangeable barrels, has a plum colored cast, which I have read sometimes happens to the bluing of guns with a high nickel content steel.
It looks like you found a nice one. Enjoy your rifle.