There is no such generic key.
If you still have FileMaker 8.5 installed on your previous computer, launch fileMaker there and select the "about" entry from the help menu. I think you'll find the correct license key listed there. Note that in current versions, you have to correctly enter the correct organization name as well as the license key. I don't know if this was also the case for 8.5 or not.
Unfortunately the previous computer was lost/stolen. This was about just under three years ago. I don't believe I'll try to see if I can come up with an organization name (I'm not sure what that would be, but I purchased it as a student, so I will try the university I was attending at the time.
Thanks for the feedback.