> Every application must have a unique signature by which the Finder can identify it. The signature can be any four-character sequence not being used for another application on any currently mounted volume (except that it can't be one of the standard resource types).
> To ensure uniqueness on all volumes, you must register your application's signature by writing to: Macintosh Technical Support Mail Stop 3-T Apple Computer, Inc. 20525 Mariani Avenue Cupertino, CA 95014
So one had to correspond by mail to get a unique application ID issued manually. Makes sense, considering the technical limitations at the time, but it makes me wonder whether other PCs/OSes had a better solution. Also, I see that the application approval process has always been a pain to this day..