For a partition $��$, denote by $N_��$ the nilpotent linear operator of Jordan type $��$. Given partitions $��$, $��$, we investigate the representation space ${}_2{\mathbb V}_��^��$ of all short exact sequences $$ \mathcal E: 0\to N_��\to N_��\to N_��\to 0$$ where $��$ is any partition with each part at most 2. Due to the condition on $��$, the isomorphism type of a sequence $\mathcal E$ is given by an arc diagram $��$; denote by ${\mathbb V}_��$ the subset of ${}_2{\mathbb V}_��^��$ of all sequences isomorphic to $\mathcal E$. Thus, the space ${}_2{\mathbb V}_��^��$ carries a stratification given by the subsets of type ${\mathbb V}_��$. We compute the dimension of each stratum and show that the boundary of a stratum ${\mathbb V}_��$ consists exactly of those ${\mathbb V}_{��'}$ where $��'$ is obtained from $��$ by a non-empty sequence of arc moves of five possible types {\bf (A) -- (E)}. The case where all three partitions are fixed has been studied in [3] and [4]. There, arc moves of types {\bf (A) -- (D)} suffice to describe the boundary of a ${\mathbb V}_��$ in ${\mathbb V}_{��,��}^��$. Our fifth move {\bf (E)}, "explosion", is needed to break up an arc into two poles to allow for changes in the partition $��$.