When was bip39 introduced https://telegra.ph/Producing-Custom-Bitcoin-Addresses-A-Take-A-Look-At-BIP39-Strategies-06-28 Though it works by using BIP32 system you could even now import BIP39 seeds into Electrum wallet. Also apart from BIp32 There exists BIP44 version which some wallet use to the era of seeds. For words that only have three letters, there's no 4th letter. https://thebookmarkplaza.com/story20700062/bip39-copyright-not-known-details-about-bip39-generator-https-www-youtube-com-channel-ucupxfytj0fb78sq55hu6ocw