I agree that it does not look right. A well known double rifle builder told me that "everyone does this" when I asked what that stuff was on my new double rifle. I have never seen this stuff on any double, either shotgun or rifle. I guess it is to cover up a sloppy soldering job quickly and cheaply. There was a bit of it on the side of the quarter rib as well. In fact, I've never seen this stuff in any photo of this maker's rifles.

I figured I was getting a weasel answer. Thanks for the help and confirmation.