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.
Find more "creative repairs" here: