Each piece has a slot (the groove) cut all along one edge, and small outcrop (the tongue) on the opposite edge. Two or more pieces thus fit together.
... Intelligence Directorate[?] Special Security Service[?] Israel ha-Mossad le-Modiin ule-Tafkidim Meyuhadim (Mossad) Sherut ha-Bitachon ha-Klali[?] (Shin ...