A broken staff is a very common item of repair. The hardened steel pivots of the staff are brittle and break on a sharp shock, such as when a watch is dropped.
This watch is such an early style movement. However it is in a later style case; a case designed for two case screws. Unfortunately, rather than drill a hole in the inside edge of the case, someone filed down the pin so it would not be in the way. As a result the movement is held in by the stem and one screw only.
Fortunately it fits pretty snugly.
You never know what you will find in a watch. They are all different, all individuals. "Creative repairs" are one of my favorite topics on this blog. More "creative repairs" here!