This case is made for this Optocoupler Tester (Version 5 without battery) https://s.click.aliexpress.com/e/_c41FJqyh 

It provides a precise and stable fit, securing the device firmly in place to prevent movement during operation. A protective cap is included to shield the contact pins from accidental damage, dust, and wear when not in use.

The design also incorporates a dedicated guiding opening for SMD component testing, improving alignment and to make it ease of use.