  Let $\mathcal{M}$ be a moduli space of polystable rank 2-bundles bundles with
fixed determinant (a moduli space of $\mathrm{PU}(2)$-instantons) on a
Gauduchon surface with $p_g=0$ and $b_1=1$. We study the holomorphic structure
of $\mathcal{M}$ around a circle $\mathcal{T}$ of regular reductions. Our model
space is a "blowup flip passage", which is a manifold with boundary whose
boundary is a projective fibration, and whose interior comes with a natural
complex structure.
  We prove that a neighborhood of the boundary of the blowup
$\hat{\mathcal{M}}_{\mathcal{T}}$ of $\mathcal{M}$ at $\mathcal{T}$ can be
smoothly identified with a neighborhood of the boundary of a "flip passage"
$\hat Q$, the identification being holomorphic on $\mathrm{int}(\hat Q)$.
