On which finger had frodo the ring, when gollum...

[Sharkû]

It took a while, but I finally found the definite answer to the old question of this topic:

"My father named the penultimate finger (the 'fourth finger' or 'ring-finger') the 'third finger'; so Frodo's 'third finger was missing' (RK p. 229)." (HoME IX, 1, V, note 9)