Roof corner cap.
Fits road side front and curb side rear. Imagine the trailer hooked up to a tow vehicle on US roads for the proper cap placement. You could also call this cap left hand front and right hand rear.
This cap fits two corners. If you were to draw an X in the center of your roof, each end of each line of the X would reach to the same cap.
Measures 12 inches long by 4-3/4 inches wide. Sold individually.
Installation instructions can be found here
Part number 4702L023