I've bumped into this very brief analysis in Structural Function of Harmony by Schoenberg and I've found that strikethrough V. Now, the chord is an D diminished seventh chord, leading to the dominant and the symbol seems the one used for secondary dominants, despite the (fallible) fact that I know one usually would write II instead of V, since we are in C major.
Where am I wrong? Is there a kind of Schoenberg's notation I'm missing? I searched through glossaries but I found nothing.