All the stock Mopar M.C. from the early 1960s came with the rubber retainer on the tip of the rod so it will not come out of the piston without a lot of effort, they don't just fall out
That being said I replace the original master cylinder in my old 1070 Bracket Duster and didn't replace the rubber retainer on the rod with a new one, I had tore the old one up pretty badly prying the rod out of the piston so I discarded it through ignorance
That car still had the stock brake light switch and bracket on the pedal assembly, I had worked on the cars sixty foot times enough to finally make the swinging pedal weight bend the brake light bracket back far enough for it to slide the rod out and hang straight down off of the stock bolt resulting in no brakes
Luckily I was at LACR which has a big uphill long shut down area so by down shifting and swerving the car around to scrub speed (it would run right at 130 MPH in the 1/4 mile back then) off I made the last turn off safely
Be safe, make sure you can stop safely all the time
Happy New Year to All