using System;
using System.Linq;
using System.Collections.Generic;

public class Test
{
 
	public static void Main()
	{
         var  List1=new[] {"123456", "432978", "321675", "321675"  };
          var List2 = new[] { "7674543897", "1234568897", "8899776644" };

            var match = from str1 in List1
                        join str2 in List2
                        on str1 equals (str2.Length < 6 ? str2 : str2.Substring(0, 6))
                        select str2;
            foreach(string  s2 in match)Console.WriteLine(s2);
	}   
}