I am beginning to document special markings on 6S281 prior to disassembly. I want to be sure and get everything possible and get it correct. I believe the picture clearly shows D. S. O which I believe indicates Detroit special order, correct me if I'm wrong please. It is the symbols that appear to be C – 3 that I am uncertain about. Is it a C or is it an O or is it something else? Does anyone know what the symbol means? Thank you very much everyone.