From what I've gathered -
The Loom will take a converter.
The Basic will not take a converter. (I have one of these).
I had the Faber-Castell BASIC with carbon fiber, and now a BASIC with Leather.
The Faber-Castell provided converter WILL fit, it was snug in the carbon fiber, but it fits fine into the leather version.
Other brands of standard international, may or may not fit depending on how thick they are, but if they are at least as thin as the one shown here (same one I'm using in the leather version):
Then it will fit.
Also Faber-Castell claims that their design has not changed to make it impossible to use a converter. (email back in May 2014):
Dear Mr. Blessing,
Thank you. The Design converter 148785 will fit into the BASIC Fountain Pens. I am working with my web team to have that detail added.
I have confirmed with my colleagues in Germany, that there was a bad production batch of smaller barrels at one point. This is what's causing all of the confusion. If the pen user finds themselves with the pen that will not allow the converter and they wish to use one, we will take care of the problem.
If you have any additional questions, please do not hesitate to contact me directly.
If it won't fit in your BASIC, you have a bad batch.
We were just playing around with a Schmidt K5 converter and a Faber-Castell Basic yesterday.... while it technically fits, it's really tight from an internal diameter perspective within the barrel, to the point where the converter gets lodged in the barrel of the pen after you take it back apart (instead of staying friction-fit in the grip). So I can definitely see where the FC converter would make more sense. Fortunately it's the same price ($5) as the commonly available standard international converter. None of the lower-end F-C pens come with converters, at least from the US distributor.
You can tell I'm doing some research for a specific reason
The converter labeled #1 in the first picture above is the same $5 one I purchased from Goulet Pens, it will not fit in my Leather BASIC, the back of the converter is too thick (and definitely didn't in the Carbon Fiber version which was more snug than the Leather one).
Edited by KBeezie, 06 August 2014 - 12:03.