That's my answer; neat!
This is a contentious issue for musicians. You're right that beat 3 is a
i chord in second inversion; it has
C–E♭–G, obviously a C-minor chord, and that should be
But let's imagine that the
E♭–G–C on beat 3 didn't exist, and we just had a half note of
D–G–B. We'd label that
V, right? Well, this notation just shows that the
V chord really "begins" on beat 3, we just have some accented non-chord tones up above. The
C is a type of 4–3 suspension, the
G already fits, and the
E♭ is basically an accented passing tone. Especially when we have that
G in the bass, our ears are already conceptualizing that beat 3 as a dominant chord. (It also helps that this particular resolution is one of the most common things in common-practice harmony; we call it the "cadential six-four.")
So to show that this is just an ornamented
V chord, we go ahead and label it with a
V Roman numeral, even though technically that beat 3 has all the notes of a
i chord. Roman-numeral analysis tries to show how a chord functions, and since beat 3 is really the start of the dominant, we go ahead and use a
V Roman numeral.
The superscript numbers just show how the upper voices are moving. The
6–5 indicates that a sixth above the bass (
E♭) moves to a fifth above (
D); likewise, the
4–3 shows the movement
Other methods of analysis would be:
I64–V, as you said. This is very common, but it fails to show how beat 3 is already the dominant.
I64–V, but with a bracket below, labelled
V, showing that both of those chords are really functioning as dominant. This, in my opinion, is the clearest approach.
PS - Something very similar happens in the next-to-last measure.