Model, of Secret Mortice and Tenon, combined with a Mitre carpentry joint

Model, of Secret Mortice and Tenon carpentry joint, combined with a Mitre carpentry joint, joining three beech-wood blocks: stamped with ‘60’ and ‘67’; over-laid in ink and pencil-marked with ‘67’, by Polytechnisches Arbeits-Institut J. Schröder Atkiens-Gesselleschaft, Darmstadt, Hesse, Germany, 1868