it is also not required that a rosary have a centerpiece, many patterns for knot rosaries, just have a knot for the 3-way join.
This is how they are made, the ones for Rosary Army. This is how I was taught to make them.
I also have an old (1950-something) book about the rosary, the author was an older man. He was in the 1st World War, & he said that back then, all the rosaries that he had ever seen were made with what he called a “little bit of metal” as a center. It was not a medal of any kind, just a metal join for the connection between the drop & the decades. I have a feeling that that would make the rosaries of those days more like the knotted kind.
Of course, it is possible to add a medal to any rosary.