Likely 'Cast Steel'; either Huntsman hot-rolled crucible steel or Bessemer process. To form a barrel, a sheet was folded over a mandrel and the long edge hammer welded.



'DD' could be Jean-Baptiste Delcour-Dupont of Nessonvaux, but the gun looks to be mid-1800s and I don't know if he was working for Pieper that early.
MLs were made for export in Belgium even after 1900 however.

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