Bip39 Checksum. Consequently, to back up the received bitcoin, you would have to transcribe all of the private keys generated every now and then. Actually, this Answer is quite unpleasant. For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, https://worlds-directory.com/listings13379072/bip39-seed-phrase-https-linktr-ee-bip39-details-fiction-and-bip39-word-list-pdf