The mark "Tes Zayin" is used in order not to write the two first letters of a holy name, which are Yud and Vav (even though many names begin with these letters). The holy name is not used regularly by us but does appear in the Tikunei Zohar. Thus, the custom of writing "Tes Zayin" is a late custom which originated in the last few centuries. In all of the old printings of the Gemara (including the Venice edition), the letters Yud-Vav are used for 16.

In this case, the letters represent a pneumonic written by Rashi before there was any concern for writing Yud-Vav and therefore he uses the letters Yud-Vav for 16. He is not the only one; the Ba'al ha'Turim also frequently uses Yud-Vav (see Devarim 6:3 and 32:42), see also Chasam Sofer (Shemos 25:20), Tosfos (Bava Basra 27a), and Rabeinu Chananel (Yoma 26b). The Maharil also uses Yud-Vav in his Hagadah, and numerous other Rishonim and Acharonim use the same letters.

Yoel Domb