Neither File Has a Version
If the key file of a component being installed (copy-A) has the same name as a file already installed in the target location (copy-B), the installer compares the version number, date, and language of the two files.
If neither file has a version number, the installer uses the logic illustrated by the following flow diagram to determine whether to replace all of the installed files belonging to the component. Because the installer only installs entire components, if the installed key file is replaced, then all of the component's files are replaced.
See the examples of default file versioning in Replacing Existing Files.