Select edges across vertices with multiple explicit normals. For imported CAD geometry, these usually correspond to the original iso lines.

selectNormalsBorderEdges.ms

(
	local compilerParams = dotNetObject "System.CodeDom.Compiler.CompilerParameters" #(
		getDir #maxRoot + "\bin\assemblies\Autodesk.Max.Wrappers.dll",
		getDir #maxRoot + "Autodesk.Max.dll")
	compilerParams.GenerateInMemory = true

	local compilerResults = (dotNetObject "Microsoft.CSharp.CSharpCodeProvider").CompileAssemblyFromSource compilerParams #(
		"using System;
		using Autodesk.Max;
		using Wrappers = Autodesk.Max.Wrappers;

		public class MeshNormals {
			private static readonly uint UNDEFINED = 0xffffffff;
			private static readonly IGlobal Global = Autodesk.Max.GlobalInterface.Instance;
			public static void GetNormalsBorderEdges(IntPtr emesh, IntPtr bitarray_pointer, bool includeOpenEdges) {
				var mesh = ((ITriObject)Wrappers.CustomMarshalerTriObject.GetInstance(string.Empty).MarshalNativeToManaged(emesh)).Mesh;
				var edges = (IBitArray)Wrappers.CustomMarshalerBitArray.GetInstance(string.Empty).MarshalNativeToManaged(bitarray_pointer);
				var edgeList = Global.AdjEdgeList.Create(mesh);
				var normals = mesh.SpecifiedNormals;

				if (includeOpenEdges) mesh.FindOpenEdges(edges);
				else edges.SetSize(mesh.NumFaces * 3, 0);

				for(int e = 0; e < edgeList.Edges.Count; e++) {
					var edge = edgeList.Edges[e];
					if (edge.F[0] != UNDEFINED && edge.F[1] != UNDEFINED) {
						int E = (int)mesh.Faces[(int)edge.F[0]].GetEdgeIndex(edge.V[0], edge.V[1]);
						int reverseE = (int)mesh.Faces[(int)edge.F[1]].GetEdgeIndex(edge.V[1], edge.V[0]);
						if ((normals.GetNormalIndex((int)edge.F[0], E) != normals.GetNormalIndex((int)edge.F[1], (reverseE + 1) % 3)) &&
							(normals.GetNormalIndex((int)edge.F[0], (E + 1) % 3) != normals.GetNormalIndex((int)edge.F[1], reverseE))) {
							edges.Set((int)edge.F[0] * 3 + E, 1);
							edges.Set((int)edge.F[1] * 3 + reverseE, 1);
						}
					}
				}
			}
		}"
	)
	local meshNormals = compilerResults.CompiledAssembly.CreateInstance "MeshNormals"

	fn selectNormalsBorderEdges obj includeOpenEdges:off = if isKindOf obj Editable_mesh do
	(
		local edges = #{}
		meshNormals.GetNormalsBorderEdges (refs.getAddr obj.baseObject) edges includeOpenEdges
		setEdgeSelection obj edges
		OK
	)

	selectNormalsBorderEdges $ includeOpenEdges:on
)