I'm going to agree with Ham on this. The evidence for it being a greave is much stronger than the evidence for it being a vambrace. furthermore if you look at the main plate of one of these things sideways on you will note there two distinctive curves or "bumps". The bigger one near the top should fit over the lateral aspect of the calf very nicely, the smaller curve at the bottom would go over the lateral malleolus (ankle bone).
Although they mainly cover the side of the leg, the front is not completely unprotected as the narrow strip connected by mail should partially cover the shin. In addition these greaves would have been worn over sturdy leather boots.
Museums have often got things wrong in the past, the Askeri Muze or Topkapi Museums in Istanbul (I presume the photos came from one of these museums) are no exception.
The one in the RA, although now separated from the cuisse, is still labelled as a "Greave".
