What format is this wallet stored in?

Question

Background

Some years ago (2017) I must have found a way to 'serialise' my wallet, because I stored the following in my password manager:

• Name: "Bitcoin wallet"
• Password: (typical generated password)
• Generation seed: (BIP39 mnemonic)
• Receiving address: (typical bitcoin address)
• Notes: (looks like base64 data, about 3000 characters in length)

I vaguely remember having the Bitcoin-Qt client installed.

Question

What method did I use to serialise (or export, or back-up) my wallet in 2017?

Extra information

What I tried so far:

• Copied the data to text file
• Ran bitcoin-cli importwallet "/path/to/file.txt" (via Bitcoin-Qt's console). Got the error "Only legacy wallets are supported by this command (code -4)".
• Decoded the text file using cat /path/to/file.txt > base64 -d > /path/to/file.dat and ran bitcoin-cli importwallet "/path/to/file.dat". Got the error "Only legacy wallets are supported by this command (code -4)".
• In Bitcoin-Qt selected File, Restore Wallet..., and selected the .dat file. Got "Wallet file verification failed. Failed to load database path '~/Library/Application Support/Bitcoin/wallets/Test'. Data is not in recognized format."
• Ran file /path/to/file.dat and got file.dat: data (I was hoping it would be identified as Berkeley DB, since I read here that wallet.dat files are BerkeleyDB database files).
• Checked the links in the answer to this question about the 'wallet export' format, but it looks like that is just a way to encode a private key, which would not result in the kind of data I have stored in my password manager.

What I did not do yet:

• Install the version of Bitcoin-Qt current in 2017, and attempt to restore the .dat file.

Answer 1

You could be dealing with a format from a Blockchain.com wallet. The Base64 string you're referencing, around 3000 characters in length, is a reasonable length for a wallet backup, which would contain multiple keys and transaction metadata