@la https://youtu.be/_OkaX7ZTM8U?t=129 at least in this old "How It's Made" it looks like some hyper-specific CNC machine? I'm guessing the cheapness is just economy of scale
@la this manufacturer has the keyway broacher as a separate machine from the other steps in manufacturing the cylinder https://youtu.be/7hJIz5Mq49Y?t=194
@la found something! it's a broach https://www.youtube.com/watch?v=b0h4ohgc730