I found this rather obscure BBS program for the Commodore 64, written back in 1986 by Charles Van Lingen, while rooting around archive.org.  Although the program disk is labeled as v4e, the program itself shows v4c as you can see from the screen shots.  The program is mostly written in BASIC, so it's highly customizable and might be useful for learning purposes.  One thing to note as pointed out in the archive.org posting is that there is likely a backdoor in the system that allows the bypassing of passwords.

Here is a link to the Toronto CBM mailing list from the peson who uploaded this to archive.org.  There is some additional information in the thread you might find interesting.