Here is a list of common devices using a DE 9 connector that I found for you. Maybe it will help.

http://pinouts.ru/connector/9_pin_D-SUB_female_connector.shtml

Now, notice the original Macintosh mouse and pre-Plus Macintosh serial connector are both on there. Also the Apple 300/1200 baud modem cable and Apple Laserwriter Appletalk connection. They have to be related to this mystery in some way. My guess without seeing it is it was the cable that was needed to connect the 5P to the Mac.