This is the 5-line business version of the Western Electric 500. The 555 had 6 buttons in front, for usually up to 4 phone lines, an intercom, and a hold button. This 1978 variation, with converted modular handset jack, has four additional buttons mounted on the right side. They were most probably for direct intercom calling to other phones. Perhaps the business that used this phone had 5 phone lines and so could not use one of the line buttons as the intercom; or maybe they just didn't want to have to dial the intercom extensions. These key sets are made for use with a 1A2 key system and have thick 25-pair line cords and plugs, however they can be used with a plug adaptor on regular phone lines — you just don't get the ability to put calls on hold, and the line buttons do not light.
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