Some Rotring Fountain pens have cult status. nowadays Many fountain pen lovers bemoan the fact that Rotring was bought in 1998 by Sanford (or Newell/Rubbermaid) which also bought Parker, Waterman and Papermate. Rubbermaid continued to sell old Rotring fountain pens until the stock was depleted. It also produced some new and cheaper fountain pens (like the Freeway, the New Orleans Series, the Impact, the Rive, the Rivette, the Altro, the Core, the Initial, the Lissabon and a few others). Some of the newer pens were made of plastic, though quite a few were also made of aluminum. Quality suffered, but some of the Sanford Rotrings are not bad. You can easily recognize the Sanford incarnations by the fact that they usually carry names (often of cities) rather than numbers. An exception seems to be the Rotring Esprit, a successor of the Rotring 400) which was sold since 1992 (but redesigned in 2004 as a telescoping pen). The old Esprit line did not have a telescoping fountain pen (but only ballpoints and pencils). I consider the Esprit one of the finest pens Rotring ever made.
Some of the Sanford pens took their cue from the old Rotring, like the Newton (which many people now call a "600" which it is not), and the Altro which is a plastic version of the Rotring 900 (and which some clown on the German eBay site tries to sell for big bucks (or Euros) as the real thing). Others may still have been designed by Rotring, and then sold by Sanford, but since catalogues are not easily available and information is sparse, this is difficult to determine. Eventually, Sanford gave up making fountain pens altogether (and for a while did not even produce any of the famous drafting pencils (300, 500, 600, 800).
I would say that if you want a genuine Rotring pen, don't buy anything made after 1998. If you want something that is just "more or less" genuine, you might go as far as 2004. Why 2004? In 2003, Guido Klamt, who oversaw sales for thirteen years at Rotring (and Sanford), finally quit. That is the terminus ad quem, as for as I am concerned.
The terminus post quem for Rotring fountain pens is probably 1982, as there really are no fountain pens from Rotring before that date. One of the reasons for this is that Pelikan owned a 50% share of Rotring since 1970, and that they probably did not want competition for their own lines of fountain pens. As they went into bankruptcy in 1982, they had to sell all their shares of Rotring. So, 1982 was the date at which Rotring became independent.
One of the first pens was probably the Rotring Renaissance which actually is essentially the same pen as the Reform 125. The innards are identical in the Reform 125 and the Renaissance (and even the outside is very similar). The Renaissance is actually a cheap Schulfüller, but it goes for a lot of money on eBay in America, just like the Gehas and Reforms.
In any case, Rotring's history of fountain pens was relatively short, that is, strictly speaking from 1982 to 1988 and at most from 1982 to 2004, or roughly twenty years.
1. Since this story is largely conjectural and has many holes, I would welcome any enlightenment about the details or the general outlines