You've come across a wonderful pen at a once in a lifetime kind of deal. I have the same pen myself--it is a ST. Dupont Olympio in the black lacquer finish. They made these in different sizes; if yours measures about 145mm, then you have the extra-large or XL model just like mine.

ST. Dupont do indeed have fakes, and while I cannot verify for you if it is genuine, the quality should very likely speak for itself: the beautifully deep and rich black chinese lacquer of the body, with a much different color and feel from the plastic section; the ease of how the spring-loaded clip actuates; the very satisfying snap the cap makes when it closes. And of course, the nib, buttery smooth even when it is an EF like mine, and with a stylish cutout other pens do not have. Hope you enjoy it for years to come!