You shouldn't use NMHDR.readSTRING(8), but NMHDR.readDWORD(8)
Besides, you can always check the value read from a position in a DataBloc object:
JScript code:
var Byte1 = DataBloc.readDWORD(0);
var Bute2 = DataBloc.readSTRING(0);
Debug.Trace(Byte1);
Debug.Trace(Byte2);
Then check if the output is what you expect.