It's normal for the 10-shot guns.
The chambers on either side of the chamber that is under the hammer actually overhang the forcing cone on each side. When the chambered round is fired, the gasses that squirt out through the barrel/cylinder gap scrub the rear surface of the forcing cone consistently, EXCEPT where the chamber mouths on either side overhang the forcing cone. In these areas, you get a slight buildup of residue. Here are a few photos of my 10-shot 617, which is kinda filthy right now (I know, I'm a bad gun owner, yada, yada; I'll clean it one of these days...).
Showing the overhanging chamber from one side:
And the pattern of residue it leaves:
It cleans off just fine; I use a stainless-steel toothbrush-style cleaning brush. That makes short work of any/all powder fouling and lead build-up.