var list = new List<string>(new[] { "ss1610a", "ss1610e", "ss16401009d", "ss164010012a" });
var newlist = list.Where(d => d.Length >= 10 && d.Substring(2, 8).ToCharArray().Any(value => Char.IsDigit(value))).ToList();

var list = new List<string>(new[] { "ss1610a", "ss1610e", "ss16401009d", "ss164010012a" });
var newlist = list.Where(d => d.Length >= 10 && d.Substring(2, 8).ToCharArray().All(value => Char.IsDigit(value))).ToList();

var list = new List<string>(new[] { "ss16401001c", "ss16401001b", "ss16401001a", "ss16401002a", "ss16401002b", "ss16401002c" });

var data = list
    .Select(d => new { d, i = Convert.ToInt32(d.Substring(9, 1)), j = d.Substring(10) })
    .OrderBy(d => d.i)
    .GroupBy(d => d.i)
    .Select(d => d.OrderBy(x => x.j))
    .ToList();

foreach (var result in data)
{
    foreach (var item in result)
    {
        Console.Write(item.d + " ");
    }

    Console.WriteLine();
}

Console.Read();