Is some sort of engravable plate. It looks like it has been there since the gun was new. It is thin and neatly inlayed into the stock.
Perhaps the gun was intended as a prize for a competition or something.
I really hope some of the more educated folks will chime in about the gun's history.
It does have very cool sights as well.
The front sight has rotatable inserts. Just turning a knob while pushing a keeper allows you to choose from four different front sights. A standard globe and inserts would be more practical but this one is very cool.
The rear sight is not only a peep but also an open sight as well.
I suggest you do a search for Mike Driskill's great post on Diana 50's. That post illustrates these exact sights much better than I can.