Comment 6 for bug 736011

ProductSeriesNavigation.traverse() call ProductSeries.getRelease()

    def getRelease(self, version):
        for release in self.releases:
            if release.version == version:
                return release
        return None

The ProductSeries,getRelease or ProductSeriesNavigation.traverse() could:

        return getUtility(IProductReleaseSet).getBySeriesAndVersion(series, name)

where series is wither self or self.context. None is if there are not matches.