Let me help you
The ring that Arthur was looking at was clearly displayed on Leo's finger, but it wasn't just any ring. The ring had been given to Leo by Quinn before he left. It was unknown to both of them that the original owner of such said ring, was standing in front of Leo right now.
Of course, the ring could have belonged to anyone, after all, his wasn't the only ring in existence. They were quite common for vampires, but there were a few things that made Arthur sure that it was the one that belonged to him.
First, the man in front of him was a regular vampire, regular vampires had the advanced tier ring. It didn't make sense to give them such a high tier ring that blocked out the sun permanently. The only people that had these, were vampire knights and above.
As for the second reason, this one was more obvious, it had the mark of a skull and a hammer. The marking showing it belonged to his family.