Writing the family name of the Chasan/Kallah/Witnesses in the Kesuba:
One is not to write the family name in a Get and so is the custom. Nevertheless, some Poskim rule that by a Kesuba, in addition to writing the names and the father’s names of the Chasan and Kallah, one is to also write the family name in the Kesuba. Likewise, the witnesses are to write their family name in addition to writing their name and the father’s names. Nonetheless, the widespread custom is not to do so. According to all, the Kesuba remains valid even if the family name was not written by the Chasan or Kallah or witnesses.
 Rama 129:16 “The family name is not to be written at all, whether it is in Hebrew or a foreign language and so is the custom”
 Igros Moshe Even Haezer 1:178; Orach Chaim 3:40-20; Seder Kesuba Kehilchasa 6 footnote 31 that so was custom of Rav SZ”A to write the family name in parentheses; Rav Yaakov Landa of Bnei Braq
 The reason: As in large communities it is possible that another person holds the same name as the Chasan or Kallah.
 Igros Moshe O.C. ibid
 Seder Kesuba Kehilchasa 6:23; Seder Kiddushin Unisuin [Farkash] 22:10; Nitei Gavriel 28:14
 Igros Moshe O.C. ibid that doing so is simply an advantage and is not required from the letter of the law.
The reason: As even in a large city it is unlikely that there exists a couple who shares the same me, both the Chasan and Kallah and their fathers. Certainly, if the witnesses do not write their family name it remains valid, as their signature is distinguished enough to recognize them and not confuse the names with other people. However, regarding a Shetar Chov, the last names are required from the letter of the law in order to be allowed to collect property line. [ibid]