identifying a mark on a silver ring

hi, im trying to identify a mark on a silver ring that i have. it has the initials R D C followed by the lion (similar to the glasgow mark) followed by a castle (flat turret with two smaller turrets on each side, followed by an S. I cant find those marks in that order in any silver marks book or online! wondered if you had any idea! tried to take a photo but couldnt get the camera to focus on the marks!:smiley:

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