This is a beautiful, beautiful pen, and it's clearly of that era (as per Dickkooty2).
Congratulations on a work of art that that should give you many years of writing pleasure.
Now for a bit of nerdiness...
I have a Mk20, a Mk30, a Pelikan 20 Silvexa, and a Pelikan 60 rolled gold. I also have a metal-bodied cartridge-filler (no idea of the model).
These five models all look exactly the same, at first glance - but there are two differences: size (length) and nib-shape.
The Mk20, Mk30 and the Silvexa all have the same nib, which looks just like that on your Pelikan 100.
The cartridge-filler and the Pelikan 60 have the same shape of nib as each other, which is different from yours.
The Mk20 and Mk30 are the same size as one another (134mm, capped).
The Silvexa, the 60 and the metal-bodied cartridge-filler are the same size as one another (132mm); very slightly shorter than the Mk20 and Mk30.
What does this suggest about your specific pen?
My guess is that the Mk20 and Mk30 are earlier models (I'm sure I've seen a Mk10 somewhere), and that the 60 and metal body are a later design. I'd guess that the Silvexa is somewhere in-between; a transitional model between the earlier design (same nib as on the Mk20&30) and the updated design (same size as the 60 and the cartridge pen). The plastic on the Silvexa feels slightly lighter than the plastic of the other two plastic bodied pens; the Mk20 and Mk30.
If forced to bet, I'd say that your pen is the pinnacle of the earlier pens (i.e., rounds off the Mk10, 20, 30 series) and thus predict that it is of the larger size. I haven't looked, but the ruettinger website is often a good source on Pelikans.
So that you can see what I'm talking about, here is a photo of these five pens - apologies for the poor image quality. From bottom-to-top:
1. Metal cartridge-filler (unknown model, very light), steel nib
2. Pelikan 20 Silvexa, 14k nib
3. Mk20 (silver-coloured trim), 14k nib
4. Mk30 (gold trim), 14k nib
5. Pelikan 60 rolled gold, 18k nib