As Chef said, the plate on the right elbow is riveted on.
The elbow wing itself looks like a normal wing with a really extravagant flair. My theory (which is only that) is that the upper edge of the flair ended up pretty thin after all of the work and grinding and it would have been exposed to more abuse since it is sitting up right where people can hit it.
They added a plate onto the upper half to reinforce it.
Even this plate (by the time it is done) will be a pretty bizzare piece of metal work. It will likely get a flute that has to sit on the corresponding flute in the wing and often the edge is lipped over the edge of the wing - to add even more stiffness (probably) and so that you don't have that unsightly look of 2 layers of steel

Putting all the shape in the left elbow add on is not all that hard (just hit it), but getting it right......
If done right, both the wing on the right and the add on on the left will sit perfectly flush with the upper and lower cannons of the vambrace when the arm is fully bent. Since the upper and lower are not the same shape, this gets interesting....
It is this kind of detail that makes really good reproductions time consuming and expensive. I bet Chef is spending some real money on this (as an armourer I hope so for Jeff's sake

)
Wade