The No. 28 ballpoint belongs to Montblanc's second line, recognisable by the two gold rings at the lower end of the cap. The refill is deployed by a lever mechanism. The model was made between 1959 and 1969 and offered in four colours: black, burgundy, dark green and light grey.
With the refill, care is needed. Today's Montblanc refills do fit in terms of length with an adapter, but the refills used at the time had a thinner tip. When the manufacturer changed over to the tip diameter now standard, it offered a front section with a larger bore free of charge as a replacement part for ten years. Anyone who let that deadline pass without being given a new front section when buying refills now owns a handsome but barely usable display piece. Before fitting a modern refill you should therefore check that its tip runs freely through the bore of the front section; front sections forced open are damaged beyond repair.