This is a Montblanc 20 in coral red – but not a “taper cap” design, which means that it was made in Germany and not Denmark. Import restrictions on German items started in 1933 in Denmark, which meant that Danish variants of the Hamburg-made pens had to be slightly different in design to be identifiable as originating at the Copenhagen factory. This pen was probably made around 1940.
The 20, like the 25, 30, and 40, had a “push knob mechanism” which works similarly to a button filler. The difference is that the end of the pen is not a blind cap hiding a button. The end is unscrewed and then acts as the button itself. This mechanism is somewhat complicated, involving a left-hand thread inside the cap, and a threaded button inside a threaded housing, as shown below in a drawing based on a cutaway photo in Montblanc in Denmark 1914-1992 by Holten and Lund.
When unscrewed for filling, the cap looks like this:
As can be observed from the photos, this pen has an extra-fine #4 nib, and is made of celluloid and ebonite. The diameter of the top of the cap has a a slight spread, but that happens to many of us after 80 years or so…
Total weight:14.1 g
Body weight: 8.6 g
Total length (not posted): 108.6 mm
Total length(with cap): 123.0 mm
Barrel max diameter: 10.7 mm
Cap max diameter: 12.1 mm