Please take my word for it this once that the pen flexes past 2.3mm. Full size (5 1/4), full flex, black chased hard rubber vintage fountain pen with an aftermarket medium (m) 18kvintage nib. The unbranded pen got a massive upgrade, going from a semi-flex, slightly rusted stainless steel nib, to a new old stock (nos, was in an unopened box of 1920's or 30's plexor nibs when i received it) 18k full flex nib with no strain on the tines and no wear to the nib's tipping. It's a quality nib right out of the box. The nib is much less fragile than if it has seen use for 100 years.
Listed is a 3rd tier(unbranded) bchr pen from the 20's combined with a high quality 1920/30's plexor 18k nib 1 fantastic pen at one heck of a good price. This item is made of 14k, bchr and black chased hard rubber.