So mystery was solved the other day in the Vintage Scope group For years people had been posting pictures of rear Unertl scope mounts with an off piece machined into it. I certainly had no idea what it was for. Lot of wild guesses. Then we found out with a picture some one took in a museum and an old fellow chimed in to explain.
“..it’s for parallax. On the Litschert scopes there a big screw that locks the parallax adjustable objective. That is an attachment that allows you to focus the scope without having to adjust your shooting position. It’s actually a pretty slick set up“