I just received my L-plates for the Nikon D800, D800E (same for both) and Canon 5D Mark III.
Both are the usual excellent Really Right Stuff quality, with perfect fit.
Even if you are not tripod-mounting the camera, they provide protection to the camera body, e.g. steadying the camera on a rock or similar hard surface. They are one reason my used cameras tend to look like new.
But the main function is for mounting in a dovetail clamp as found on most ballheads; the “L” shape allows quick and easy mounting in either horizontal or vertical position (instead of flopping the tripod head over, never a convenient or ideal choice).
B5D3-LA for Canon 5D Mark III — there is also a bottom plate only as well as a version for use with the optional Canon battery pack/grip BG-E11.