According to my Merc manual that is the correct part number. Part is listed as: Bracket assy (oil pan)-rear for 1949-51 Mercury. Also listed with part number is: Bolt (Hex Hd)-
5/16"-24 x 3/4" - brkt. to flywheel housing cover assy.
You could make one without too much trouble, I think. Flat piece of steel with a 90 degree bend, kind of like an angle iron bend with a shape to fit the oil pan that you can see in the photos. Click on photo to enlarge.