This blue Tachikawa pen holder is slightly larger (slightly thicker and longer) than the pink and white Tachikawa holder. A nice grip if you have larger hands.
The attachment point of the nibs is made of plastic. Two different sizes of calligraphy nibs fit in the holder. In any case the G nibs (except titanium) and a Brause EF66 fits in the small attachment point.
There is a cap with the pen that you can slide over a nib. This allows you to leave the nib in the pen while traveling.