If real, and if the material turns out to be chalk, paint, graphite or ink, in my mind it is likely a marking related to its removal from inventory at whatever Miami shop did the work. “X” meaning “removed,” “R” possibly “repair”, “revenue” or “inventoRy”, and “O” possibly meaning “zero” or “out.” Those familiar with aircraft parts/materials inventorying will have better ideas I am sure.
How tall are the letters?
Thanks,
Jon