Silver box
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An Autovon telephone keypad with the four precedence levels
A silver box is a modified DTMF keypad that adds the four additional keys used in the now-obsolete autovon phone system. This was usually accomplished with the addition of a toggle switch and a crystal that switched one column of a standard phone keypad into the "fourth column" used in the Autovon system. (These keys were used to set the priority level of a call, and are the red keys in the photo on the right. More information is available in the Autovon article.
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