Thanks, Mikael, for the remark, but there are others, too, who have very detailed knowledge, especially in aircraft archeology - which I am not specialised in.
Here is the drawing of the Hs 129B's belly. Although the above piece of metal skin
looks like originating from a Hs 129, I cannot readily identify it - hence my post here, on the forum (supposing, of course, that the dwg. is accurate):
