  We study singularly perturbed linear systems of rank two of ordinary
differential equations of the form $\hbar x\partial_x \psi (x, \hbar) + A (x,
\hbar) \psi (x, \hbar) = 0$, with a regular singularity at $x = 0$, and with a
fixed asymptotic regularity in the perturbation parameter $\hbar$ of Gevrey
type in a fixed sector. We show that such systems can be put into an
upper-triangular form by means of holomorphic gauge transformations which are
also Gevrey in the perturbation parameter $\hbar$ in the same sector. We use
this result to construct a family in $\hbar$ of Levelt filtrations which
specialise to the usual Levelt filtration for every fixed nonzero value of
$\hbar$; this family of filtrations recovers in the $\hbar \to 0$ limit the
eigen-decomposition for the $\hbar$-leading-order of the matrix $A (x, \hbar)$,
and also recovers in the $x \to 0$ limit the eigen-decomposition of the residue
matrix $A (0, \hbar)$.
