Jordan-H\"older decomposition of regular $(a, b)$-modules

A classical result of singularity theory states that the spectrum of an isolated hypersurface singularity is symmetric with respect to $n/2$, where $n$ is the dimension of the enclosing space. We prove a similar result for the Jordan-H\"older composition series of the $(a,b)$-module associated to an isolated hypersurface singularity.