I have got the message that the extra letters in the clues spell out, and I have completed the grid.
Does anybody think that something more has to be done in the grid?
I suspect that there is something because it would be too easy otherwise. Or maybe I am just looking for an extra task to be completed wwhen there isn't one!
Although I've not made a start on this one yet, the preamble does not suggest any highlighting or alteration of the grid is required in this particular case. Sorry to disappoint you !
These 4 are surely thematic content in a puzzle commmemorating itself. There is nothing in the preamble to indicate we should act on any message hidden in the grid.