The mark consists of the words "STRONG BOX" written above a drawing of a strong box, the strong box having an upper rectangular lid, a lower rectangular base, and a smaller rectangular central face, a vertically oriented hasp having two-fifths darkened region in the upper left portion which has an arcuate termination on its right end, with a padlock securing the hasp, the padlock dominating the central portion of the center section of the box, being largely darkened with some light shading and having six rivets positioned on its face, a centermost rivet supporting a trapezoidal closure plate which is tilted to the left to reveal a keyhole directly below the centermost rivet; a darkened region of the lid of the box occupying two-fifths of a left hand portion with an arcuate termination on its right end; a pair of concentric rectangles defining a band extending around the center face of the box, the band having a plurality of rivets equally spaced therein about its peripheral extent; a triangular darkened region of the left portion of the inner concentric rectangle, the point of the triangle extending downwardly; a narrow light stripe and dark stripe extending parallel to the hypotenuse of the triangle; a darkened region of the base of the box occupying one-fifth of the left portion and having an arcuate termination on its right end.