לָאו דְּיַעֲקֹב הוּא is not Yaakov’s signature. That is, it is known for a fact that there is no one living in the city where the document was written who is named Ya’akov and whose signature matches the signature on this document. Therefore, the court will recognize that the signature must have originally stated: “So-and-so, son of Ya’akov, witness,” and that the last line had been excised, and they will invalidate it.
וְדִלְמָא בִּשְׁמֵיהּ דַּאֲבוּהּ חֲתַם לָא שָׁבֵיק אִינִישׁ שְׁמֵיהּ דִּידֵיהּ וְחָתֵים בִּשְׁמֵיהּ דַּאֲבוּהּ Rami bar Ḥama challenges: But perhaps this witness signed using the name of his father instead of his own name, as a gesture of respect toward his father. The Gemara answers: This is not done; a person does not discard his own name and sign using only the name of his father.
וְדִלְמָא סִימָנָא בְּעָלְמָא הוּא דְּשַׁוְּויֵהּ דְּהָא רַב צָיֵיר כְּווֹרָא רַבִּי חֲנִינָא צָיֵיר חֲרוּתָא רַב חִסְדָּא סָמֶךְ רַב הוֹשַׁעְיָא עַיִן רָבָא בַּר רַב הוּנָא מָכוּתָא לָא חֲצִיף אִינָשׁ לְשַׁוּוֹיֵהּ לִשְׁמֵיהּ דַּאֲבוּהּ סִימָנָא Rami bar Ḥama challenges further: But perhaps the witness made this name into a mere distinguishing mark that he uses as his signature, as it is known that Rav used to draw a fish as his signature mark, rather than signing his name, and Rabbi Ḥanina used to draw a palm branch as his signature mark, and Rav Ḥisda used to sign just the letter samekh, and Rav Hoshaya used to sign just the letter ayin, and Rava bar Rav Huna used to sign his name by drawing a ship’s mast [makhota]. The Gemara answers: A person is not so insolent as to use his father’s name as a distinguishing mark.
מָר זוּטְרָא אָמַר לְמָה לָךְ כּוּלֵּי הַאי כׇּל מְקוּשָּׁר שֶׁאֵין עֵדָיו כָּלִין בְּשִׁיטָה אַחַת פָּסוּל Mar Zutra said: Why do you need all this? Why go to such lengths to answer the question posed above? There is a simpler answer: Any tied document whose witnesses do not end on a single line is not valid. The Gemara had previously assumed that the witnesses sign one after the other, beginning at the document’s bottom line and going upward toward the first line; this arrangement leaves open the possibility that the signature of the first witness could be truncated by an unscrupulous party. Mar Zutra explains that this is not so; rather, the signatures are written with each one beginning opposite the bottom line and heading upward toward the beginning of the document. Therefore, if a line of text is excised from the bottom of the document, the names of all the witnesses on the reverse side will be truncated, and the forgery will become apparent.
אָמַר רַב יִצְחָק בַּר יוֹסֵף אָמַר רַבִּי יוֹחָנָן כׇּל הַמְּחָקִין כּוּלָּן צָרִיךְ שֶׁיִּכְתּוֹב וְדֵין קִיּוּמֵיהוֹן וְצָרִיךְ שֶׁיַּחֲזוֹר מֵעִנְיָנוֹ שֶׁל שְׁטָר בְּשִׁיטָה אַחֲרוֹנָה מַאי טַעְמָא § Rav Yitzḥak bar Yosef says that Rabbi Yoḥanan says: There are two halakhot with regard to documents: For any erasures in a document, the scribe must write at the end of the document: And this is their verification. That is, he must list the erasures, stating that on line so-and-so there is an erasure and a correction stating such and such, for each erasure. And the second halakha is that the scribe must review some of the details of the document in the final line of the document. What is the reason for this second requirement?