These mufflers were called venturi mufflers and a number of engines used them when mufflers first started to be used. I remember my old Taipan 61 had one. There was concern about losing power with a muffler so the idea was that the airflow from the prop would funnel down through the hole and cause a low pressure through the centre which would drag the exhaust through the slots which you might be able to see if you look through the hole.