The parallel rails have a double tee cross section and the leg of the rear T fits into slots in the sides of the frame. When the frame is welded, the rails are captured with the front slots ahead of the frame so menu strips and price modules can be mounted from either end and will mesh with each other...http://www.google.com/patents/US4521984?utm_source=gb-gplus-sharePatent US4521984 - Menu board frame and insert
