Chris:
I am NOT a photo collector or expert.
However I did a Google search for 1951 Mantle photos and the first ones shown in both home and away uniforms ALL show the patch I referenced on the left sleeve.
See:
https://www.google.com/search?q=1951...&bih=549&dpr=1
Granted, the photo used in the 51 Wheaties is there as well as are many others that are obviously post 1951.
Fred