remove and replace. The wiring system employed wires terminated with crimped spade lugs and slotted-screw terminals; most electrical connections in the phone could be disconnected or reconnected with a standard screwdriver. Originally, the line cord and handset cord were secured by screw-down terminals at both ends, with a strain relief anchor. Both cords had matching colors, but most color sets initially were produced with neutral, dark gray cords, with exception of the ivory and brown sets. Starting in 1955, all sets, except dark blue were equipped with color-matched cords as dyes and manufacturing methods became stable. By 1973, line cords were changed to a neutral gray color and from a round to a flat cross-section. As a customer option, telephone sets could be ordered for portability with four-prong plugs, but most sets were hard-wired to wall connection boxes. In the 1970s, much smaller modular connectors were introduced for both ends of the line cord as well as the handset cord to facilitate standard connection methods for third-party equipment within the FCC registration system. Development Several telephone models were derived from the basic model 500, using some of the same components. The model 554 was a wall-mounted version. Other special-purpose models included additional features. This included phones with dial lights (500U), party line sets (501), keysets (540 and 560 series), call directors, panel phones (750 series), industrial and outdoor phones (520 and 525), and automatic dialers (660). Starting in 1963, the Dreyfuss design adapted itself well to the newly announced Touch Tone® service. For the trial period, the dial was replaced with circular closure containing a keypad and a telephone number label. The 1500 and 2500 models adopted a more rectangular design style; the curves of the original design gave way to raised borders forming a lip around the finely-ribbed faceplate, in the middle of which the keypad and number window was centered. Standard touch-tone telephones generate twelve DTMF signals in a grid of three columns and four rows of buttons. In contrast, some special purpose telephones are equipped with a fourth column of keys for the A, B,C, and D DTMF signals for service priorities in the AUTOVON system. Original sets used a transistor oscillator; newer phones use an integrated circuit. The model 1500 only had push buttons for the ten digits, while the model 2500 used 12 keys and included the '*' and '#' DTMF signals to allow for