RM/COS used . as a path separator. I think this is from OS/360. It makes sense to me in that member selection in C also uses '.'. Yeah, from the OS/360 wiki:

RM/COS used . as a path separator. I think this is from OS/360. It makes sense to me in that member selection in C also uses '.'. Yeah, from the OS/360 wiki: "The file naming system allows files to be managed as hierarchies with at most 8 character names at each level, e.g. PROJECT.USER.FILENAME. This is tied to the implementation of the system catalog (SYSCTLG) and Control Volumes (CVOLs), which used records with 8 byte keys."