  Given a complete, positively filtered ring $(R,f)$ and a compatible skew
derivation $(\sigma,\delta)$, we may construct its skew power series ring
$R[[x;\sigma,\delta]]$. Due to topological obstructions, even if $\delta$ is an
\emph{inner} $\sigma$-derivation, in general we cannot ``untwist" it, i.e.
reparametrise to find a filtered isomorphism $R[[x; \sigma, \delta]] \cong
R[[x'; \sigma]]$, as might be expected from the theory of skew polynomial
rings; similarly when $\sigma$ is an inner automorphism. We find general
conditions under which it is possible to untwist the multiplication data, and
use this to analyse the structure of $R[[x;\sigma,\delta]]$ in the simplest
case when $R$ is a matrix ring over a (noncommutative) noetherian discrete
valuation ring.
