Many thanks for your questions.
The Building now occupied by Dorothy Perkins was built in 1880. The architect’s plans show a shop occupying the ground floor. The front part of the first floor was occupied by two reading rooms and the new museum was in a room to the rear. The collection at his time would have been comprised largely of private collections donated by members of the Berwickshire Naturalists Club (founded 1831). A new Art School (it had been part of the Corporation Academy now occupied by the Leaping Salmon in Golden Square) was on the second floor. The Art school was run by James Wallace, father of the better known James Wallace jnr.
The museum moved to Berwick Barracks about 1985 and the library to the Parade Holy Trinity School where it remained until moving again about ten years ago to a new building shared with Northumberland County Council Social Services.
As to your second question, I have never heard of such a thing but nothing would surprise me. Can you be more specific? Might the “occult marks” be in fact masons marks? These were symbols carved by each mason to indicate how many stones he had worked on so that he could be paid. These are usually visible externally only though. Some good examples exist on the walls at the south end of the Quay Walls, in Lord’s Mount and the West Gun Tower at Berwick Castle.