I've owned several 50mm f/1.8 Nikkors over the years and they all had an unmarked detent at f/2, and that is how it meters as well.

I don't think it would be that difficult to make a lens with 1/3 stop increments. My Pentax screwmount lenses have 1/2 stop indents at most settings, and I know at least one Nikkor lens that had 1/3 stop settings: the 105mm f/4 bellows lens.