I'd be highly surprised that C5 mobs would eas the pain with shoulder external rotation - but who knows stranger things have happened - My knowledge of Wikipedia reference-linkfacet joint and disc somatic referral patterns (from early research) suggests the cervical spine doesn't refer in that way unless its a Wikipedia reference-linkradiculopathy and then its usually more lateral than Kings area and lots of other signs would exist.

King is the pain aggravated when external rotation reaches 90deg or immediately when the shoulder is externally rotated. And by flexing the arm to 90deg is that to the front to side. At this point is sounds like a shoulder pathology but more info would be needed. Can you carry heavy things without pain; can you sleep on that shoulder; can you press on the pain with your finger; any noises with shoulder; can you reach up your back ok; can you do a push up or press up from a chair using your arms. etc etc.