I believe that the OP's left mirror bracket is for a '51 and later trucks with vent windows and, as already mentioned, upside down. If a '47-'50 left side bracket were mounted upside down, you would be able to see the mirror through the windshield (but wouldn't be able to anything behind the truck).

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission