Abstract base class with multiple subclasses. Some subclasses have additional properties in DetailXXX tables. DB lacks FK relationships. Relationships are mapped on "natural" composite keys.

FNHMappingExample.cs

// Component is an abstract base class
// Weld inherits from Component
// Weld has additional properties in DetailWeld table

    public class ComponentMap : ClassMap<Component>
    {
        public ComponentMap()
        {
            Table("Component");
            CompositeId().KeyProperty(x => x.ItemId).KeyProperty(x => x.RevNo);
            // Properties mapped here
            DiscriminateSubClassesOnColumn("ComponentTypeId", 4);
         }
    }

    public class WeldMap : SubclassMap<Weld>
    {
        public WeldMap()
        {
            DiscriminatorValue(8);

            Join("DetailWeld",
                 wd =>
                     {
                         wd.KeyColumn("ItemId");
                         wd.KeyColumn("RevNo");
                         //More properties mapped here

                     });
        }
    }

Yes on swapping the order of calls to KeyColumn.
FNH is 1.1 binary (1.1.0.685)