Here you can see the various parts of a pen and the names for each part.
This may help when we’re talking about your pen.
The nipple on the back of the housing is the part that the converter fits to. I’m often asked to make pens with vintage nibs to take converters but the problem with this is that they don’t have a nipple. The nipple helps with flow by creating capillary action from the converter into the feed to the nib. If you cut the nipple off the feed above and tried to write with it, you would experience some skipping and poor performance. The feed nipple inside the housing nipple is very important to flow.
The barrel shoulder, or the step, is what the cap lip seals against.
The inner shoulder is where the lip of the section seals inside the cap. This helps keep it from drying out. On some models the cap lip is where the seal is made.
Problems with your parts.
If you commonly remove your nib unit for cleaning be very careful not to over tighten. Doing so can possibly stress the housing and create a small crack. There may be instances where the housing is never removed but still cracks. Sometimes these things just happen, but luckily, not very often.