The IBM Model M keyboard uses a buckling spring mechanism to achieve the classic clicky feel and audible feedback it's famous for. A spring is located under each key and mounts onto a pivot plate (aka hammer/platter/lever/flipper) via a tiny post, but it's difficult to attach the spring to the hammer, especially if the keyboard is fully assembled. This simple tool makes it easier to install the spring.
Start out with the key and spring removed from the keyboard. One end of the rod is slightly concave, and that side goes into the hole and sits on top of the tiny post pointing up from the pivot plate. Slip a spring over the printed rod, then use the plunger to push the spring down over the post. Once you remove the plunger and rod, the spring should stay well attached to the post.
Print with fine detail (e.g. I used .08mm layer height on a Bambu P2S) with the rod laying down and the plunger standing upright. Material shouldn't matter much - I used some PLA I had lying around.
For more info on Model M repair, an amazing resource can be found at https://www.ardent-tool.com/keyboard/Model_M.html, and spring-specific info at http://sandy55.fc2web.com/keyboard/model_m_trivia.html
The author marked this model as their own original creation.