Interesting to know that yours fit. I emailed Pablo about the standard #5 JoWos fitting an Eco this morning. His reply was no, they do not. I'm looking for an architect grind, and I'm tempted to just but the entire pen to get it.
That was his advice to me, too: the curvature on the TWSBI nibs is (I think) customised for them to sit just a *little* more snugly against the feed, so that the standard #5 JoWo nibs may be a bit loose. Glad to hear that's not always the case - but any buyer of these nibs needs to be aware that fpnibs.com aren't guaranteeing their suitability for use in TWSBI pens.