LMC is incorrect in its listing of the same left-side mirror-arm for 47-55. There were two different arms, due to the addition of a driver's side (left side) vent window added in 1951.
Here are photos of the different styles of miror arms (from Classic Parts). Originally, they were all painted lack - ignore the chromed finish - just look at the bolt/screw orientation to the arm.
47-55 right side mirror arm (passenger side)
47-50 left side mirror arm (driver side)
51-55 miror arm (driver side)
