Hello: I recently purchased a bunch of what was labelled as Parker 21 parts on eBay. A lot of different caps, barrells, and sections in various colors. I was going to put them together, including buying any needed additional parts, into whole pens and sell them. I thought that Parker 21 sections and barrels were interchangeable. However, I ran into a problem as shown in the photos. The photos are of a blue and maroon parker 21 sections and barrels. The maroon barrel is too small to fit over the blue section and the blue barrel is too large for the maroon section, and won't thread. The blue converter says parker 21 and the maroon only says parker. So, are the different parker 21 version barrel and sections not interchangeable, if so, which versions are shown here, or are the marron parts not parker 21 parts. Thank you.