Here is mine:
As you can see, there are small plastic ribs that are at either end of each spot for a pen. The distance between these ribs is approximately 14.5 cm. Also, the slots allow for pens with a body circumference of slightly less than 12 mm.
You can squeeze in a few more millimeters if the pen can fit slanted slightly like the Bic Crystal[1] in this image:
The interior length is approximately 15.8 mm without the ribs. You could hypothetically shave these ribs down a little bit to give more room without the need for slanting but that may affect the structural integrity. It probably would not affect too much but I have not tried it.
Overall, I really like these for expandable storage of items that I do not keep out in a regular display case. My only problem with it is only allows for longer pens. However, for $15 on Amazon, I will overlook this.
Fifth from the left. ↩︎