The 251 propelling pencil was made from 1971 to 1984 and the matching 281 ballpoint from 1971 to 1979. Neither belongs to the 1950s, as is occasionally supposed; both are classic products of the 1970s, and within the Montblanc range of the day they sat in the middle of the quality ladder.
Company history is the key to valuing them: Montblanc only positioned itself consistently as a maker of luxury goods at the start of the 1990s. Before that the range ran from the plain school pen to the Meisterstück, and the middle series sold in large numbers. Pens of that sort seldom appreciate to any appreciable degree; in pen collecting too, that is reserved for the handful of especially sought-after models and editions. Anyone who inherits a set like this will usually get more out of using it as an everyday writer than out of waiting for it to gain in value. The manufacturer gives reliable information on authenticity; datings of older models offered by shop staff, on the other hand, are best treated with caution.