I'm not entirely clear about what you're asking...
The hammer strut/spring/housing unit is held in by it's own spring tension when it's put in place in between the detent on the lower and the pocket/recess on the hammer.
Not sure what happened to the plunger set you pictured in the first post but I believe those are not 'disassembleable' as the tube holding the strut and spring on that type appears to be swaged over the end of the spring after the spring and strut are assembled into it. Did yours come apart somehow so there's three separate parts? If it's still all together, does the strut compress (with considerable tension) into the spring tube/housing and spring back again smoothly? Maybe it just wasn't seated properly in the the pocket where it rides in the hammer or on the other end the detent in the lower? Does the pocket/recess on the hammer look worn/wallowed out?
There's also an alternate version (metric maybe) that used an open ended shorter tube and the strut/spring is uncaptured until it's in place between the hammer and lower. I think the two types are interchangeable but not sure. I'd assume that's the type your replacement spring would go into...