Mandrel-less thimble. This is made from .025 brass, so it's quite thin. Helps to keep the weight down on the rifle.
Seriously, most original thimbles I've seen are very thin.
Note that the ribs don't go all the way around. This makes for easier inletting.
To do this work, you need PITCH, available from RIo Grande, or some such jewelry supply. Heat the pitch with a torch, and then set your metal in it. To remove your piece from the pitch, pass the torch over it. Everything just loosens up. Heat the piece, wipe clean with a rag.
First step, punch ribs in from the back:
The blank with raised ribs and the swell punched up from the back. Not pinched and soldered together yet.
Now it's soldered together, and the dome is raised more to my liking:
Now the dome is detailed from the front with small punches. The thimble is filled with pitch to support the metal.