I know virtually nothing about this other than it's supposed to be US MP issue.
Came from a local surplus shop that actually does get a lot of surplus. 23 inches closed; 36 inches open.
Correct use avoids hitting the head supposedly with legs and arms being primary targets.
https://www.milsurps.com/images/impo.../CLiEmwf-1.jpg
https://www.milsurps.com/images/impo.../PqASxtD-1.jpg
https://www.milsurps.com/images/impo.../P4KJKHU-1.jpg
https://www.milsurps.com/images/impo.../tt0JYfs-1.jpg
https://www.milsurps.com/images/impo.../cLUcQ2w-1.jpg
https://www.milsurps.com/images/impo.../VgdQlhR-1.jpg