Michael Gardi (the same guy who made the Think-a-Dot "computer" toy replica, and the Dr. Nim game replica) made a 3D model of the mechanical Digi-Comp II marble computer so anyone can 3D print one of their own. "I now have the complete collection E.S.R. Inc. products!" says Michael.
From his Instructables page:
Digi-Comp II is a mechanical computer invented by John Thomas Godfrey and manufactured by Education Science Research (E.S.R., Inc.) in the late '60s. As can be seen in the pictures above, the device consists of a frame (about 14 by 28 inches) propped up at an angle. Computations are performed by balls rolling down channels on the top of and inside the platform through gates. Some gates are fixed switches that merely redirect the ball down a particular channel while others are flip-flops that both redirect the ball and change the state of the flip-flop in the process.
Intended as an aid for teaching computer concepts, the Digi-Comp II can count, perform basic arithmetic, and obtain either the "1's" or '2's" complement of a number. The device can be run in auto mode where the balls are released automatically after each step of an operation until the operation is complete, or in manual mode where the user initiates each step.
Along with the STL files and instructions for this project, you will find a PDF of the manual that came with the original Digi-Comp II. I brought this file to a local copy center and had them print me the spiral bound booklet you can see above and I'm really happy with the results. Also there is a PDF of the parts and assembly instructions that you will need to complete this Instructable.
This offering is a full size working replica of that Digi-Comp II from 5 decades ago. I have tried to make it as authentic as possible based on the images and technologies that I have available to me. The original top and inside decks for instance were vacuformed plastic attached to pieces of masonite board, and the flip-flops and switches were injection molded. In my replica the decks and pieces are all 3D printed. Also I do not have an original Digi-Comp II to work from. I am indebted again to Jaap Scherphuis (he did the original video you can link to above) for his invaluable assistance in providing me with good quality images. Finally I did modify the design of some of the flip-flops (that are not visible when operating the device) to improve their reliability.