Thanks to Francis Goossens (aka fountainbel on FPN) for the brilliant repair!
He turned a new complete celluloid ink window insert and drilled the old barrel out to receive the insert. The insert goes deep into the barrel such that the piston travels in one seamless chamber. Francis also cut inner section threads and outer 4 leads cap threads are manually on a lathe.
In addition, he replaced the plastic piston head with a cork one, and calibrated the piston mechanism so that the user no longer needs to torque the piston head against the section for the piston mechanism to engage. (See Francis’s post on FPN explaining the telescoping piston mechanism in these 1950s Montblanc models.) Having to do that over decades, combined with the degradation of the celluloid with use and with time, most likely led to the breakage you see in the “Before” photo.
The 1950s 146 pens are great writers, and Francis’s efforts should lengthen the service life of this one by several years, hopefully decades if I’m careful.