GuidSteganography.cs

public static class GuidSteganography
{
    public static Guid Hide(this Guid id, short value)
    {
        byte[] span = id.ToByteArray();
        int i = value * 16 - 15;
        span[7] = (byte)(span[7] + i);
        return new Guid(span);
    }

    public static short Reveal(this Guid id)
    {
        int a = (((Convert.ToInt16(id.ToByteArray()[7]) ^ 0x0F) + 15) / 16);
        return (short)(a < 4 ? a + 12 : a - 4);
    }
}