That was the Storm Boat Motor lower unit. It was shipped with Speeditwins, Speedifours, and Big Fours in 1946 and 1947. They shipped a model that was absent the "wings" on the gear case in ’48. Yours have been cut off, but it isn’t a factory job. That is a VERY RUGGED unit, It is also a couple MPH slower on a light boat. All parts are interchangeable between the three models.
The fastest gear case was the pre-WW2 model. The 1949 – ’50 case is a close second.
