No. 0 Lever Filler
This pen is from the personal collection of Ariel Zuñiga - a generous contributor on FPN, seminal Montblanc collector’s books, and now this website. Watch out for a series of reviews of truly rare pens thanks to AZ’s generosity!
In terms of size, Collectible Stars I shows examples of lever fillers in OO, O, 1, 2, 4, and 6. The size O example we have here measures a tiny 10cm. Look at it dwarfed by the 139 in this picture!
Rösler, J., & Wallrafen, S. (2014). Collectible Stars I: 1908 - 1947.