Hi Similia!!
At one book i had read that is a w.
The first letter of Waite. It seems that is comunn the author put the first letter of the name at one ace.
And another book i read that is a M. That means mercy, the people that drink fron this cup will get a lot of mercys.
Well, i belive more at the second. or the 2, because Waite, proves that he was a wize man, maybe he saw the 2 things.