This is a single tube ballpoint pen styled after the Majestic series of Pen State Industries.
The bushing sizes are documented on the instructions, but the actual measurements of the fittings and tubes are listed below.
The fittings are both 0.495"
This pen is only available at Penn State Industries
When assembling the pen, remove the finial cap, and remove the transmission before pressing the finial clip on the upper end of the tube. Then reassemble. Do not put the refill into the pen until you have completed pressing the nib and cap into place.
If you have to dis-assemble the nib or clip, use a transfer punch to remove the fitting. The fitting is composed of several pieces that may separate when being driven out. If this happens, drill a 9mm or 23/64" hole in a scrap piece of acrylic and insert the tube portion of the fitting into the hole. Press the fitting back together.