That plate on the frame is not a patch/repair. That is a splice plate, straight from the factory. I'll bet there is one just like it on the frame on the opposite side of the trailer. It's a "C" channel frame made up of two or more sections on each side and spliced together with splice plates and huck bolts.
See exampleBy the way, those pre-punched holes aren't there to save weight. Those have a folded innerward edge, they are for strengthening the frame so it doesn't fold over in the middle like folding a slice of bread.
