Does anyone know if the purpose these symbols has ever been deciphered?"The longest of the [DSS Biblical] manuscripts, which is known as the St. Mark's Isaiah Scroll ... is made of strips of leather, stitched at the edges to form a continuous scroll. It is about a foot wide by 24 feet long. Although it shows wear, it has been carefully repaired and its condition may be counted good. The text is in Hebrew and its 54 columns contain the book of Isaiah in its entirety. There are symbols** in the margins which have not been deciphered [in 1956, not sure when or if this has ever been done]; perhaps they were indications in connection with the use of the scroll in Public Worship.
** Symbols in the margins of the columns on some of the Scrolls, the meaning and purpose of which are unknown [in 1956]. Possibly, they were indications of a liturgical nature for use in public worship. On the other hand, certain scholars have rather shockingly suggested that they are just "doodlings."