Collectors refer to the "reddish" appearance a "patina".
It's an "ageing" process. Like an aged wine, or bottle of fine Scotch...
(in otherwords, it ain't bad!!!). Only an option on "older rifles...well taken care of....
I have a similar era Marlin M336 in .35Rem that looks just like yours, execept for the Leupold VariX-2 2x-7x scope it wears...
It too, shoots and functions perfectly.... close to MOA with a "warm" handload.
Since aquiring it in the late '90's, it's taken about 30 deer....
One of my favorite "hunting" rifles...
Longest shot was about 210yds. Bang-flop. But then, I don't zero for 100yds. I sight +3" at 100yds for ~-4" at 200yds...
It was essentially a point and click shot. I missed the first two shots and deer stood looking away from me where bullets impacted beyond him. I thought I was shooting under until I realized- "dummy", you're shooting OVER him. Held under the top of his back and he did an underside flip and layed on his back and waved good-bye with his front hooves...(broke his back at the shoulder...).
The .35 shoots flatter than you've been lead to believe !!!
Strange... these guns had to get to be 40+yrs old and enter the "internet" age before many, including myself, came to really appreciate these fireams... I was too caught up in ballistics charts and the "latest-greatest" back in the day....
At least now I can afford to own and use these "oldies but goodies"....