How to restore wallet from 15-word seed mnemonic? The wallet was created around Feb 2013, using unknown wallet software. All that is known is a passprase, and 15 words. I suspect the passphrase was used to encrypt the wallet, and is probably irrelevant here, since the wallet file itself is lost.

All words are found in the "old" Electrum list. But the number 15 is puzzling, since no version of Electrum ever produced 15 words by default, as far as I know. I considered the possibility that this mnemonic was extended with custom words, but it looks like custom words in Electrum work only with the "new" word list.

I cloned Electrum repository from github and tried converting mnemonic to hex seed using the old list, then converting it back to mnemonic using the "new" word list. This method produces 15 words that Electrum GUI does recognize as 12 + 3 (extended), but leads to empty wallet.

My working hypothesis: there ought to be another wallet software that existed around 2013 that used Electrum's "old" word list and generated 15 words by default. Any other ideas or suggestions?
Mnemonic phrase from 2015 or earlier, and not an Electrum seed?
Then there is a high probability it is blockchain.info's legacy wallet recovery mnemonic.
Which can be recovered here https://blockchain.info/wallet/forgot-password
Correction:
This is a blockchain.info mnemonic. It contains your WALLET ID and Password (pre 2013 contains only Password and/or password plus alias)
You can decrypt the mnemonic using blockchain.com webpage they have a legacy (HD is only 12 words) page which changes from time to time.
With that you can login to your account and access your coins.