These pen tubes have been designed for the following pen kit available here:
https://www.woodturnerscatalog.com/p/2/2750/Artisan-Panache-Pen-Kit
To see a forum post about this particular assembly go here:
https://www.shapeways.com/forum/t/panache-pen-tubes-and-assembled-pen.97048/
For additional information consult the penmaker's interest group in the Shapeways forum:
https://www.shapeways.com/forum/t/penmakers-and-3d-print-pen-making.80076/
Extra notes: These tubes have been modified as compared to the design in the original forum post. Essentially each tube was duplicated, rotated by 180 degrees, and combined into one tube. The large thin areas on the original design had a tendency to cause warping of the tubes (particularly the shorter cap tube) which caused complications during assembly. The modifications helped to eliminate the large thin areas while retaining interesting surface patterns.