I have no idea which version of Electrum I first used on my computer, but my first Electrum wallet (which I still have, albeit with no coins in it) is named "default_wallet". I don't think this has changed recently. Certainly according to the documentation here (https://electrum.readthedocs.io/en/latest/faq.html#where-is-my-wallet-file-located), the first wallet created is called "default_wallet".

Incidentally, I also found this old post of another user who appears to have had the exact same problem as OP. No mention of if he managed to recover his coins, though: https://bitcointalk.org/index.php?topic=1438110.0