This is an adjustable drag I have not seen before. When you turn the screw it bears down on the small disc which adjusts the drag. The adjustment is easily made as the screw-head is large and it is knurled. Just a slight turn makes a fairly substantial difference in the drag.
I know I have seen that "waisted" foot several times before on English reels, but can't associate it with a maker. The stud on the back which secures the spindle is also unusual, especially for what I assume is an English reel.
I've seen Slater and Eaton & Deller reels with similar notches on both sides of the foot but usually the E & Ds have cutouts in the ends, also. The click keeps me circling back toward Walter Dingley, but Fosters Bros. could also be a possibility. Those Brits copied each other a lot.
Love those Open Face Spinning Reels!(Especially ABU & ABU/Zebco)