These pen blanks are designed for the 8mm metallo pen kit available from Penn State Industries in chrome or gold finishes. The "lathe" series of blanks have slightly over-sized diameters to allow pen makers to turn their blanks on a lathe. This gives the ability to generate custom shape profiles and gain a greater control over final finished dimensions that bypasses small process variations inherent in the 3D printing process. Some blanks are also patterned to allow inlay techniques using colored liquid resins or other materials.
There is some DIY work you must do to your blanks prior to final pen assembly. These steps are basically gluing the brass tube(s) in the pen kit into your blank(s), sanding the ends of the blanks down to the ends of the brass tubes, turning your blank on a lathe to desired shape, and surface finishing your blank with a sealant or other finish as desired.
For final pen assembly it helps to use a dedicated pen press to achieve good results. Alternately you could use a large C-clamp with wood or plastic covering the clamping faces but you need to be extremely careful to not crush or damage your pen mechanism.