The wallet was created around February 2013, using unknown wallet software. All that is known is a passphrase and 15 words. I suspect the passphrase was used to encrypt the wallet, and may not be relevant here, because the wallet file itself is missing.
All words are found in the “old” Electrum list. But the number 15 is confusing, as no version of Electrum has ever produced 15 words by default, as far as I know. I’ve thought about the possibility of this memory being expanded with custom words, but it looks like the custom words in Electrum only work with the ‘new’ word list.
I cloned the Electrum repository from github and tried to convert my mnemonic to a hex seed using the old list, then convert it back to the “new” word list. This method produces 15 words that the Electrum GUI recognizes as 12 + 3 (extended), but results in an empty wallet.
My working hypothesis: There must have been another portfolio software that was around around 2013 that used Electrum’s “old” word list and produced 15 words by default. Any other ideas or suggestions?