OK, I had to go to the Internet Archive to find the original set of rules. Here they are:
Bottom Line: The "X" is used strictly for tie-breaking purposes:
"Center rings in each target are tie breaking "X" scores. In the case of "X" ties, the best "X" wins."
So, a perfect score on the Black Death Target (BDT) can only
come from putting 5 shots completely inside the 1/2" target, without touching any of the surrounding black areas.