Cap issue - Swan c.1920’s ebonite lever filler

[StayLost]

Hi there! I hope the knowledgeable folks can share some repair tips. 
 

I have a Swan ebonite pen with a cap that won’t screw in. With the section removed, the cap effortlessly and snuggly threads the barrel.
 

The inside of the cap appears to have a metal sheath of sorts, and the opening is narrower than the pen’s section. It feels like the pen at the section cannot get deep enough to engage the threads.  
 

I added a picture of the inside of the cap. Any thoughts?

 

Much appreciated.