Butler's conjecture for generated coherent systems

Let $(E,V)$ be a generated coherent system of type $(n,d,v)$ on a general curve $X$ of genus $g\ge3$, where $n,d,v$ are positive integers and $v>n$. Write $D_{E,V}$ for its dual span bundle, and let $S_0(n,d,v)$ denote the space of generated $\alpha$-stable coherent systems of type $(n,d,v)$ for small $\alpha>0$. Butler's conjecture. For a general $(E,V)\in S_0(n,d,v)$, $(D_{E,V},V^*)\in S_0(v-n,d,v)$. Moreover, $S_0(n,d,v)$ and $S_0(v-n,d,v)$ are birational. This conjecture concerns the stability of dual span bundles and predicts a birational correspondence between the relevant moduli spaces; the source gives no resolution of the general statement.