Barry,
I won't answer, since I couldn't find any answers. I'll just comment that I can see the same two William Thurbers on the 1774 census, and I thought I'd try to help you and hunt the second elusive William down -- but he sure hides well!
On a side note, I added a couple of category tags to a couple of my Rhode Island profiles for for your 1774 census project, but then I didn't quite understand the rest of the code that's needed to make them show up in your linked page, so I'm sorry to say I failed on the follow-through.
Reba