A zinc box. People says it's a box for an additional propellant charge for minethrower (mortar). It was found near SJ-3 bunker at Summajärvi. An inscription on the top: "Zusatsldg 10 cm NB W". Lower: "ACJ 39". I think it came to Finland from Germany, because 'zusats' in German means something like 'addition'. And the diameter of this box is exactly 10 cm! But I don't know about German mortars used by Finns - they used English Stocks 81 mm mortars.