I have not stripped a Lewis before but most guns with a rib extension, Fox Lc Smith etc, have the rear portion of the rib silver soldered in place. That area is under a lot of stress, hence the higher strength solder. I can likely be repaired but that would be a very big job. The amount of heat require would result in a full relay as well as possibly causing issues with the lug. I would be shocked if it was soft soldered at the factory. Good luck with it.
Steve