I tested this in CS3 and it appears to work. Magic!

So I can open a CS5.5—with Soxy— and it will then script it and save as CS5.5 rather than auto open the IDD as CS 6? More clarification please.

Thanks Jongware. That was super helpful!

Here is how to do the same kind of thing in C#:

using System;
using System.Linq;
using System.IO;
 
 
namespace InDesignWrapper {
  class VersionChecker {
    private byte[] InDesignFileHeader = new byte[] { 0x06, 0x06, 0xED, 0xF5, 0xD8, 0x1D, 0x46, 0xe5, 0xBD, 0x31, 0xEF, 0xE7, 0xFE, 0x74, 0xB7, 0x1D };
    private int byteOrderFlag;
 
 
    public string GetInddFileVersion(string filename) {
      var fileinfo = new FileInfo(filename);
      var stream = fileinfo.OpenRead();
      var reader = new BinaryReader(stream);
 
 
      var fileHeader = reader.ReadBytes(16);
      if (!fileHeader.SequenceEqual(InDesignFileHeader))
        return "Not an InDesign file";
 
 
      string fileType = new String(reader.ReadChars(8));
      byteOrderFlag = reader.ReadByte();
      reader.ReadBytes(4);
      var majorVersion = reader.ReadInt32(byteOrderFlag) - 2;
      var minorVersion = reader.ReadInt32(byteOrderFlag);
 
 
      return String.Format("CS {0}.{1}", majorVersion, minorVersion);
    }
  }
 
 
  static class BinaryReaderExtensionMethods {
    public static byte[] ReadOrderedBytes(this BinaryReader reader, int count, int byteOrderFlag) {
      if (byteOrderFlag == 2) return reader.ReadBytes(count).Reverse().ToArray();
      return reader.ReadBytes(count);
    }
 
 
    public static long ReadInt32(this BinaryReader reader, int byteOrderFlag) {      
      return BitConverter.ToInt32(reader.ReadOrderedBytes(4, byteOrderFlag), 0);
    }
  }
}