Can somebody identify the purpose of this rectangle from the Black Prince's effigy?

http://www.greenleaf-workshop.co.uk/gallery/d...fullsize=1

It looks like something is missing from the effigy.
The space between the plaques is larger there than elsewhere and there is also a circular hole in the effigy below the rectangle with no apparant purpose.
It looks like there was a separate piece attached via the rectangle and the hole which would fit in the larger gap between plaques.

Are there any early drawings of the effigy which would allow us to see if anything was originally in that space?

Judging from its positioning on the belt, and the fact that he is wearing a sword, it would seem likely to me that it once held a matching dagger.

But, I have not seen any representations of Prince Edward's effigy that would support this.

Though other contemporary effigies do seem to support this.