Vintage children's wooden construction set

Would love help in identifying this! It's a wooden children's construction set, probably from the 50s, with no brand name or any other writing on the box! Box roughly 25x15cm. Cover slides off with the blocks tightly packed inside, checked the blocks and there were no identifying marks on the blocks either. There is some kind of serial number on the bottom right: "III 12 14 KI 967/56" and also (286) on the top right. (The KI could be a K1 as well.) Google brings no relevant results with those serial numbers.