Looks like a custom shop rifle...
Who was the OEM for Winchester Custom Shop synthetic stocks?
Yes, that's a Custom Shop serial number, you could order a particular serial number as long as it hadn't already been used. The Extreme Weather model came with a glass bedded McMillan Supergrade stock. The barrels were not Winchester factory tubes but were supplied by various aftermarket makers.
I ordered a pair of left hand Extreme Weather Model 70's back in 2000, a .30-06 and a .375 H&H. They took a year to arrive. The .375 was set up like the .458 in the auction except for the muzzle brake. The front and folding leaf rear sights are from NECG.
The .375 H&H was a beauty. Both rifles have super slick actions, they feed everything including empty cases. The .375 was extremely accurate, the .30-06 not so much.
Couldn't say for sure if the threaded barrel and brake are standard or a custom option offered from Winchester. When I ordered mine they were like Honda cars, you got whatever they made the way they made it. That may have changed at some point, I was unhappy with the heavy barrel on my .30-06 - it was somewhere around a #3 to #4 contour - so I sent it back and they replaced it with a lighter barrel.
Those sights are definitely what the CS offered so you never know, perhaps it's all original or maybe just the original barrel and the brake was added later.