The graphic form of letter Տ is believed to be derived from an Iranian prototype. In ancient times, letter “tyun” had the form , similar to the modern Armenian Մ [m], but differing from it in the short left column. Afterwards, the letter acquired the present-day Տ-shaped form. Despite its similarity with the Latin S (s), these letters have different origins. Incidentally, today the existence of the Latin “double” of letter Տ and several other letters of the Armenian alphabet (O, U, L) is used in the number plates of Armenian cars, which is very convenient: such “international” numbers are intelligible for everyone.
This practice exists in other countries as well – thus, in Greece and Russia the car numbers contain only those letters, which are identical to Latin letters. As to the old form of Armenian , it has not disappeared without a trace: it is reflected in the small letter տ, which was formed by lowering of the right element. It is noteworthy that it did not escape resemblance with other Armenian letters: in some manuscripts, especially in old printed books, տ can be easily confused with small teller ա (ayb). Periodically, various attempts were made to eliminate this confusion. Thus, in the Armenian magazine "Murch" (Hammer), which was published in Tbilisi since 1889, starting from 1898–1899 and until its closure in 1907, տ was printed with a long middle line elongated downwards – like the one used today by the Armenians in writing.
The authors of the “Aramyan” font went even further (XIX century) – Janik Aramyan and the brother of the great seascape painter Ivan (Hovhannes) Aivazovsky – scholar and theologian Gabriel Aivazovsky (Ayvazian), who substituted տ for s, which differed from the capital letter Տ only in size. While “tyun”, changing its shape, was constantly in search for its graphical “I”, the appearance of its Greek “cousin” – “tau” (T) changed little. But the attitude towards “tau” varied fundamentally: from contempt to worship. The point is that in ancient times T resembled one of the forms of the crucifixion cross. This letter was treated better after spreading of Christianity, when the cross became the symbol of salvation.
See more at Aram Khachaturians "Armenian Alphabet. History and Symbolism" book