The Sampo is the most argued-over object in Finnish tradition and the argument has produced no answer. The readings offered fall into families. One takes it literally as a mill of plenty, a machine that produces grain, salt and money without input, a wish common to a great many traditions. Another, associated with Eemil Nestor Setala and much repeated since, takes it as the world pillar, the axis on which the sky turns, with the many-coloured lid as the vault of heaven and the theft and breaking as a cosmological event rather than a robbery. A third reads it as a coin die or a treasury, on the basis of trade contact with Byzantium. A fourth reads it as a shamanic drum. Others have proposed a sky-supporting tree, a temple pillar, and a device for producing prosperity that is best left undefined because the runes leave it so.
The textual situation makes the question harder rather than easier. The Kalevala is not an inherited epic but a work Elias Lonnrot compiled and shaped out of oral runes collected in Karelia and elsewhere, and the Sampo cycle is among the parts he arranged most heavily. The runes themselves are not consistent about what the object is, which may mean that the singers did not agree either, and that a single original meaning is being looked for that never existed.
The atlas records the object and the argument rather than picking a family.