I learned the technique when I had a muzzle loading English shotgun in for a rib repair. The thimbles were attached as I described. But I have also seen them as Dan has described.
On the last Hawken rifle I made, I riveted the hollow rib to the barrel, so soft soldering the thimbles on later was easy, and very strong. I use silver bearing electrical solder for this job...flows beautifully and is easy to clean up.