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The slit is there to assist in getting an accurate measurement of the length of the forearm, as the inner edge of the slit is in line with the center of the axis of rotation of the forearm. The limit switches were mounted that way to ease zeroing, as trying to get an exact angle measurement was proving difficult if one switch was mounted in the elbow, much easier to ensure that the arm is straight.
Hope that helps explain the design process and reasoning behind our decisions!