That sealant is asphaltum, or pitch, so your cases were pull-downs from loaded ammo. Nothing wrong with that. The sealant dissolves completely and pretty quickly in odorless mineral spirits and probably any of a dozen other solvents. A Q-tip wet with mineral spirits will get it out with some rubbing, but it's easier just to stand the cases mouth-down in a covered dish with half an inch of mineral spirits in it and let them sit overnight. Pull them out and let them dry the next day.
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