The ITT 601 KSU has a capacity of 5 lines and 10 phones. Its size and shape are similar to the Western Electric 551, and both are called "shoeboxes."
It had a major innovation over the 551 with an interrupter that was solid-state, instead of motorized. Unlike the 551, it had space for a dial-selective intercom system. The 551 could hold only a manual intercom which used buttons to activate buzzers.