I was in Arthurdale (just west of Kingwood) on 13 Aug 2011 and stopped by the Visitor Center. Lo and behold, down in the flower bed next to the entrance was a marker - Preston County / Maryland. The Visitor Center was closed, so I could not ask anyone about it. There are several markers like this that are missing or were just recently replaced, from the borders on Rt 7, Rt 219, and Rt 50. I don't know the original location of this one, but will do a little research.
UPDATE: This marker was replaced in 2013 at the Rt 7 / Preston County- Maryland State Line location - see Post #1382 on Page 93.
Located in the flower bed next to the entrance of the Arthurdale Visitor Center. (Yes, I was in the cage today.)
ALSO: One of the nearby markers was repainted and moved to a much better spot -
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