Hi Mike,
Unfortunatley that book shows no images of the marks, and like you, I'm drawing a blank as to who the maker would have been. The only fit as far as Grimwade is concerned is Isaac Aaron (1102), who entered marks on the 21st March 1791. Any submissions by him up to the 30th May 1791 would of course carry the 1790 date letter.
Heal offers no real alternative, but a possibility may be someone like John Alderhead, he entered just the one mark back in 1750, but was a well known retailer, I believe, for the rest of the century. Could it be his marks overstriking that of another?
Here is the link to a topic where John Alderhead's trade card can be found:
viewtopic.php?f=38&t=18763and this link shows details of the hookah:
http://en.wikipedia.org/wiki/HookahRegards Trev.