Yeah. I was hoping NOT to be the crash test dummy and that someone would know what they fit in reliably..It says compatible with Energel and Pilot G2, so it would be the same length as a rollerball, but the fit depends more on the shoulder of the cartridge. I've found that Pilot G2s will fit in a Jr pen, but even though it's the same length, it doesn't extend fully from the tip of the nib. That being said, test it in a few kits and let us know how it works. I wouldn't trust them in pens I sell until I went through a couple to see that they are consistently reliable.