I know the book is about British stuff, but does he suggest that the list and marks apply only to British RA?
The Spanish and Irish stuff is somewhat different, but does a different style mean different meaning? Couldn't that be just regional differences?
Also, Passage Tomb Art went through at least two distinct style phases at Knowth, with earlier mofifs being overlayed at a later stage (some suggestion that the stones were reused). The early motifs were incised and the later ones pecked (I think that's the right order).