The DN means the original gearbox had the Laycock-DeNormanville overdrive. The JLE also means the same.
Assuming soap and water didn’t work, I would use very ultra fine 000 steel wool to clean it.
Yes of course, carefully. This appears to be iron oxide (rust) but could be copper (brass) oxides. One could also try Turtle Wax (or your favorite brand) Polishing Compound or Brasso cleaner.