The Austrian Post put these milestones everywhere, and you can still find them on the old dirt roads which are now only used by foresters (and hikers.) I have no idea how old (or not old) they are, but ÖPT stands for Österreichischen Post- und Telegraphenverwaltung, which was what the Post called itself from 1866 until 1996.
Now that’s an interesting hybrid — half horse, half motorcycle. Wonder if it runs on petrol or hay.
It’s actually a space-saving sign saying that the area is protected for hunting, and both horseback riding and motorcycles are verboten.