It's quite an old pen and from what I've read it seems to be made in the 1960s.
The one I have looks very old too, with scratches and blemishes. The pen is made mostly of resin with several parts with metal trimmings, and the nib of course. It's quite lightweight, just the way I like it.
The pen uses a screw-on type cap with classic Montblanc white star design on top. Other than that, there's no information on the pen that identifies the make and model of this pen.
The nib is rather interesting. It's cut quite sharply and I'm not sure if it was like that originally or the result of wear and tear over years of writing. Either way, it's really smooth on paper. Ink flow is nice.
The feed is also interesting. Usually fountain pen feed have their serrations on the exterior. In this case for the Montblanc 31, it's directly beneath the metal nib. And the bottom of the feed is smooth.
The Montblanc 31 has a piston filler inbuilt. You turn the knob behind to refill ink. There's no way of knowing how much ink you've left so it's best to refill when you plan on using it a lot that day. Personally, I dislike inbuilt piston fillers when I can't see the ink level inside.
Unfortunately for me, the pen I received has a crack that grows around the diameter. Ouch! I'm not sure what happened here. It's really a fine pen but it's such a waste to have a crack like that destroy the pen. Now ink comes out from that crack. It might say something about the durability of this pen.
Overall, it's a simple and plain pen to me. It's not particularly eye-catching. If you can get it cheap under USD $100, it might still be worth it.
Below are some sketches from the pen. The lines are very predictable, as expected. I enjoy the smoothness and the lightweight quality.