This ASP Retaining clip is a factory replacement part that fits any ASP F-Series baton. If your baton will not stay closed, it is likely that you need one of these. To check; just unscrew the end cap and see if the clip is broken or mangled.
This clip can also go out of adjustment over time which can cause you to use more force to open your baton which can cause the baton to become hard to close. Most people would not attribute a hard-to-close baton with a faulty clip, but it happens.