The Hemisphere, Preface and Charleston share the same feed, and identically shaped nibs. Charleston and Preface are 18k, of course, while Hemis are steel. Each model has unique engraving. Gold nibs have the size on the bottom of the feed, steel nibs have it on the nib. Thus, it's a good idea to keep the feed with the nib on the 18k's.
I've included a Kaweco Sport nib and feed on the left because they are also interchangeable. The feed and nib are the same diameter and overall length. I use several in Hemisphere sections, in 1.1 and 1.5 mm italic. They are the easiest and cheapest way I've found to get an italic nib in a modern steel nib Waterman (topic for another post).
And below is the difference in Waterman engraving (L to R Hemi, Preface, Charles):
Edited by esteroids, 24 July 2013 - 05:57.