Beginner: combining two nearly identical classes?

How can I combine these two nearly identical classes?

  class MyMsgAAA
  {
    private int msglen = 0;
    private byte[] MyMsg = new byte[500];

    /*constructor*/
    public MyMsgAAA(byte[] ba)
    {
      //Now build the message
      this.add(0x01);
      ...
      add some bytes special to AAA messages
      ...
      //message itself (.add escapes any special bytes)
      for (int i = 0; i < ba.Length; i++)
      {
        this.add(ba[i]);
      }
    }


    /* THE REST HERE IS IDENTICAL */
    public void add(byte b)
    {
      MyMsg[msglen++] = b;
    }
    //Return message
    public byte[] msg
    {
      get
      {
        byte[] target = new byte[this.msglen];
        Array.Copy(MyMsg, target, msglen);
        return target;
      }
    }
  }

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  class MyMsgBBB
  {
    private int msglen = 0;
    private byte[] MyMsg = new byte[500];

    /*constructor*/
    public MyMsgBBB(byte[] ba)
    {
      //Now build the message
      this.add(0x01);
      ...
      add some bytes special to BBB messages
      ...
      //message itself (.add escapes any special bytes)
      for (int i = 0; i < ba.Length; i++)
      {
        this.add(ba[i]);
      }
    }


    /* THE REST HERE IS IDENTICAL */
    public void add(byte b)
    {
      MyMsg[msglen++] = b;
    }
    //Return message
    public byte[] msg
    {
      get
      {
        byte[] target = new byte[this.msglen];
        Array.Copy(MyMsg, target, msglen);
        return target;
      }
    }
  }

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In general, I want to call

MyMsgXXX(various parameters)

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If the parameter types were always different I could just make different constructors for the same class. But what if some messages have the same type of parameter (such as in example above)?