"anchor plate" in English). From the Middle Ages up until the mid 1800's, large wooden beams or logs were used in building multiple story houses and these anchor plates locked (or anchored) the beams in place. If the beams were to shift, the building could collapse. They are often made of cast iron and their dark color makes them easily noticeable against the reds, yellows and oranges of the buildings.
Click here and here to see diagrams explaing which forms come from which periods (website in Swedish, but you can see the dates).