Have you tried to import this wallet.dat to a newer version of Bitcoin core?
If not yet I suggest you try it first but make sure make a backup copy of that file first before you open it with the latest version of Bitcoin core.

Or try to open it with a notepad since you said it was not encrypted and you might see the private key when you open it with notepad.

If not, try this one below which is created in 2011 when the wallet.dat is not encrypted yet.

- https://bitcointalk.org/index.php?topic=25091.0