Thanks for your comment Fernando and happy to hear that you also have no doubts on its age. A very nice example of an astonishing preserved medieval belt you show!
I just searched the net for medieval depictions of knights, noblemen, horses etc. but couldnīt find something fitting. But what Iīve found again is this depiction of a messenger, althogh it is German and 15th century. For me this is still a possible usage of this shield. Will keep on searching