It's more likely that that decanter was made by Kastrup, pre-merger. I've never found any glass that definitely dates to after 1965 bearing the old-style Kastrup or Holmegaard labels - which isn't to say that it didn't happen. But this blue was used by Kastrup, as well as Kastrup-Holmegaard.
As for stoppers, you can find them facet-cut (around 1900), with applied rosettes (1900 - 1930), plain (1850s - present), and crown-shaped (1938 - c.2000). These are the only Holmegaard/Kastrup stoppers that I have records for. Stylistically, yours just doesn't match the decanter.