A silver box is a modified DTMF keypad that adds the four additional keys used in the now-obsolete autovon

Autovon

AUTOVON, short for Automatic Voice Network, was an American military phone system built in 1963 to survive nuclear attacks. AUTOVON was first established in the United States, using the Army's SCAN system. Around the mid-1970s AUTOVON expanded to the United Kingdom, Asia, the Middle East, and Panama...

 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

Autovon

AUTOVON, short for Automatic Voice Network, was an American military phone system built in 1963 to survive nuclear attacks. AUTOVON was first established in the United States, using the Army's SCAN system. Around the mid-1970s AUTOVON expanded to the United Kingdom, Asia, the Middle East, and Panama...

article.)

Modern phones with an integrated circuit based DTMF generator can frequently be modified by simply soldering a wire from the 1633hz leg to a switch that toggles
between that leg and the 1477hz leg for the rightmost column of keys.