This is an adapter that will allow you to use standard Schlage 5-pin KIK cylinders inside Sargent and Greenleaf 826/831/833 locks. It's common for people to acquire these locks without keys or even a medeco KIK cylinder. Since these lost are usually US Military restricted and getting replacement keys/cylinders are out of the question, they are left with a large paperweight.
This will allow you to at least put it into service. Here's a video of it in action with a 5-pin Schlage KIK (no pins in it since I didn't have a key for it) https://www.youtube.com/watch?v=O6vsxahiwbY&feature=youtu.be