What confused me was seeing both the windage and elevation scales in the same plane. But when the small one is turned 90 degrees, its scale is then correct. The actual aperture is missing.
The last time I shot 303 was at Bisley on the 400 yard range. The club that I was a member was the Ewhurst rifle club, an indoor range for 0.22 rifles and pistols. The club was a home guard post during the war and we had a large box of mostly 303 ammunition in the safe. Eventually the police officer who inspected the facilities told us that we had no buisiness having full bore ammunition at the club and would not want to see it still there the next time he called. So we sat around the table and sorted out the ammunition, from 416 Rigby to .22 shorts. Most was 303 dated 1943, over 500 rounds, so we went to Bisley and hired two Lee Enfields No 4 Mk 1 with target sights and didn't have a single misfire. The sights were a more modern design than that BSA No 9.