Strictly speaking the Newton is not a 600 at all, even though it is often, as a matter of course, referred to as a "600 new style." This makes some sense, as there is also a new Newton that morphed into the Parker Facet. The last pen seems to have a bad reputation among Rotring enthusiasts. I believe this reputation is unjustified, as it is actually an interesting fountain pen that also writes very well. But be that as it may, I don't want to talk about the different makes of Rotring here. Rather, I would like to say something about how to exchange the nibs of the Newton 600 with other nibs.
There is a lot of misinformation about what nibs the 600s take. If you search the "fountain pen network" for "Rotring" and "nib," you will see what I mean. So, let me make one thing clear from the outset. What I say does not hold for the old-style 600s. They have different nibs, and I don't know where to get these (or what other Rotring Pens have the same nib).
There are still some nibs for the Newton on the German eBay site (for about $10.00). There are also nibs for the Esprit. They also fit. I also know that the nibs of the Rotring Freeway and Esprit, available for between $25.00 and $40.00 on eBay can be freely exchanged with the newer 600 (a.k.a. Newton). In both pens, the the nib is friction fit. It easily pulls straight out from the feed section and it can easily be pushed into the feed section of other pen. I have done this several times and, in particular, have transformed medium-nibbed Newtons 600 into fine-nibbed ones. (I have not exchanged gold nibs, however, and I do not know whether the nibs have the same design.)
Other newer pens by Rotring have very different nibs and cannot be properly exchanged. In particular, the nibs of the Altro, Initial, the Rivette, the Core, the Skynn, and the Surf are different. I would suppose the same holds for the Rive (but I do not own one of those and have not checked). I am not saying that these nibs might not perhaps be jammed into the Newton 600, but only that this does not seem to be a good idea to me, that I would not do it, and that it will in all likelihood lead to problems.
To say it again, the nibs for Freeways and Esprits are interchangeable with the Newton 600. Since the pens are still to be had at a reasonable price, they can reasonably be viewed as sources for "back-up" nibs. And, don't believe everything you read on the fountain pen network on this topic.
1. The "Surf" has really nothing but the name in common with Rotring. It truly is a "Sanford." This does not mean that is not a good inexpensive pen. It only means that it is not even the last in the lines of Rotrings.