The WW II plane belongs to a friend of mine. He had already changed the beech handles and he asked me to install one of my heavy plane blades.
I dismantled the plane and saw that the sole is warped and slightly convex. So I started with filing and then with a mechanical scraper and a leveled cast gray iron plate to flatten the sole.
My blades are 4,5 mm thick and the new back iron is 3 mm thick. By these dimensions the small lever didn’t no longer touch the back iron. I have cut a “H” into my back iron and bend it so that I eliminated the gap. The installed screw was too short so I made a longer screw with a thread of 7.5 mm.
The mouth is opened only 0,6 mm and now the plane is working perfect.