Schematic illustration of our modelling approach. The main SNR shock direction is perpendicular to the line of sight as traced by OH masers, which are known to reside in dense postshock gas. The spectral line broadening, however, hints at supersonic fluid motion along the line of sight, which are expected, given projection effects and complex cloud substructure (left sketch). Our modelling decomposes the complex shock geometry in three components: one shock layer propagating away from the observer (fitting the red-shifted component of the line profiles), one shock layer propagating towards the observer (fitting the blue-shifted emission of the line profiles), and an additional layer of gas (found to correspond to postshock conditions) that extends into the line wings (right sketch). This latter layer makes up for the unrealistic accumulation of matter in the post-shock region inherent to one-dimensional shock modelling, which does not allow the post-shock gas to relax in all possible directions, as well as for existing post-shock gas created and stirred up by shocks perpendicular to the line of sight.