In the group buys Ranga usually offers CC filler with Bock nib/feed/collar group (or alternatively Jowo nib feed collar group).
That is because they can thread the section either for the Bock collar or the Jowo collar.
When you use the Bock or Jowo nib/feed/collar group attached to a converter (usually Schmidt) what you are actually using is a German modern (industrially produced) writing mechanism, surrounded by a nice ebonite body.
This provides a very reliable writing experience, but once you have more than two, three of these set ups (perhaps with different nibs) you start to realize that (besides small variations nib to nib) they all write the same.
That's all very well if you like your pens to be reliably similar.
In addition if you take off the converter and use the pen barrel as an ink reservoir you can convert these pens to ED, which provides a slightly different flow than CC, although the main flow is governed by the nib group.
When you choose the ED version, usually in the group buys Ranga offers Wality or Kanwrite nibs (Ambitious also in the past, although I have not tried one).
The ED version has the advantage of coming with an ebonite feed, which does provide a different flow than the Bock/Jowo nib groups, usually much more generous.
My experience with Wality and Kanwrite nibs has not been so great, however (just my opinion).
So despite the group buy did not provide the option, I asked for a custom modification to include a Bock nib with ebonite feed in ED version.
The ED versions also have the advantage of a more affordable price, but the different behaviour in the ink flow is what is making me like them. Of course ED versions are slightly less convenient if you want to toss the pen around (and not find the section dirty with ink when you open the pen). In such cases I use the CC versions, often in ED mode, especially the transparent versions.
Nonetheless using the ED versions (possibly with a very nice nib) really does give you the feel of what a traditional ebonite pen is all about.