Bip39 algorithm https://www.storeboard.com/myrtierisaliti Though it works by using BIP32 approach it is possible to nonetheless import BIP39 seeds into Electrum wallet. Also apart from BIp32 You can find BIP44 Edition which some wallet use for that era of seeds. For words that have only three letters, there's no 4th letter. https://penzu.com/p/2ea6922b70254307