I know there are also some cased 'Gulvvase' in black and opal that aren't catalogued, so presumably it's a one-off or short-run piece - perhaps sold through the factory, perhaps as a special order. They did do a short-lived black/opal version of the 'Palet' kitchenware series in the early 1970s called 'Sort Palet' (Black Palet), so it could have been made around the same time.
An engraved '2' on a Holmegaard piece generally indicates a production second, with some kind of flaw (very often so minor as to be invisible).