Bip39 Is Not Defined Regardless that it makes use of BIP32 system you could still import BIP39 seeds into Electrum wallet. Also other than BIp32 There's BIP44 version which some wallet use for your era of seeds. You may see the entire source code in standalone, offline html file. Alternatively https://bouchesocial.com/story20531736/old-seed-phrase-generator-detailed-notes-on-seed-phrase-generator