No. 0 Lever Filler
Here we have the tiny No. O size pen made of black hard rubber. Montblanc made lever fillers for a very short period between 1922 to 1929, and this makes them highly collectable today. Learn more about the MB lever filler at the bottom of this article --> No. 1 Lever Filler.
The nib has "trademark' imprinted on both sides, and it was made between 1919 and 1925. This particular lever design (MB had a few designs) was used between 1922 and 1926. Considering these facts, we know that this particular pen was made between 1922 and 1925.
In terms of size, Collectible Stars I shows examples of lever fillers in OO, O, 1, 2, 4, and 6. The size O measures only about 10 cm. That is tiny, even though it's not the tiniest. See how it compares to my celluloid 149 and a No. 1 Ret and No. 2 safety in the picture.
The standard line - not Meisterstück - lever fillers were more commonly available in black, but customers also had the options of mottled red and coral red (see mottled red version --> link). The mottled red pens command far higher value amongst collectors today, and the coral red even more.
Rösler, J., & Wallrafen, S. (2014). Collectible Stars I: 1908 - 1947.