Hello again, Mary,
It seems I was able to answer my own question. After a tumultuous arrival in Frederick, GBM established HQ on the farm of Dr. Lewis Steiner west of town. This is stated in Joseph Harsh's "Taken at the Flood," 237, cited to Paul E. Steiner, "Medical-Military Portraits of Union and Confederate Generals," Phila.: Whitmore, 1968.
Using period county maps and land records I now intend to pinpoint this property, perhaps even track down a possible war claim filed at the National Archives.
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