Ah...OK...it seems the 7L (which I presume is what you've pictured and not the 7D?) is a simplified version of the 7M that I played with so it doesn't have the mid range adjustment. For some reason the 7L is missing from the carb manuals at http://www.osengines.com/manuals/index.html

The problem with the carb on the engine I was given to try to sort out was that the yoke had been fitted upside down. The flats that key to the spray bar to rotate it are set at an angle which meant that the metering slots were out of alignment so it was impossible to set the idle mix. Never having seen one of these carbs before it took a while to figure out