There are slides devoted to this topic: Lecture 6b, pp. 27-34.
The Computer History Museum’s page on George Stibitz includes images and some descriptions of the Model K.
For a more extensive article, read “Relay Computers of George Stibitz” on the History of Computers site, which goes into detail on the development and design of Stibitz’s computers.
“Early Innovators: George Stibitz,” a short video from the Computer History Museum, features Stibitz describing his idea for the binary adder and its results, as well as a demonstration of the “Model K.”