The yellow stuff is old varnish, to protect the French gray. You can soak it off with solvent, re lube the breech and go from there. The butt plates are a weird organic plastic that features horse hooves and rubber, among other things, and, they all shrink. Gunter sells a polyureathane version that wont't shrink, but, doesn't have the color styrations that you sometimes get with the originals.
Do not shoot your gun with a loose forend. This is bad juju. PM me and I'll walk you through it. it isn't tough, but, you need decent screw drivers.
Best,
Ted
