One of those not-often seen pieces of vintage phone equipment. I believe this is called a fuse / lightning arrestor. They also served as connection blocks for various lines within a subscriber's circuit. White porcelain base with all parts present and accounted for, including original mounting screws for the block and the two insulators that come with it. I presume it is in good working order and was simply taken out of service when a new line was brought in. Nice go-with or make your phone system a vintage working display. $20 post paid in US.