Uhm...okay. would never have guessed it. 
I never would have either.
And I was prepared to give massive hints until:
Kweepa proved to be a genius and solved it without any!
That was proof that the riddle was pretty solid and could be solved as it stood.
And Kweepa told me in PM that it was a nice "Aha!" moment when solved, which was what I had hoped for, so I decided to just let the riddle stand for a while until the actual solution was released so that maybe others could have that same kind of fun moment.
Sorry if this annoyed anyone in the meanwhile.