I Think I Figured Out the Original Context of the Marcosian Interest in the Number Six

Yes, the Hebrews always believed that the sixth letter was involved in the creation of the world. Yes, there were a number of interesting traditions related to the 'sixthness' of the universe but one thing I always overlooked - and it is very important when considering the Alexandrian Church. The Hebrew lunar calendar has 354 days; the Alexandrian Church shifted to a 360 calendar (cf. Clement Strom.i.24). In other words, six days were added to the traditional calendar of the Jews or as we see in Maimonides:

By how much does the solar year exceed the lunar year? By approximately 11 days. Therefore, whenever this excess accumulates to about 30 days, or a little more or less, one month is added and the particular year is made to consist of 13 months, and this is the so-called embolismic (intercalated) year. For the year could not consist of twelve months plus so-and-so many days, since it is said: throughout the months of the year (Num 28:14), which implies that we should count the year by months and not by days.[Maimonides Mishneh Torah Sanctification of the New Moon Translated from the Hebrew by Solomon Gandz, Yale Judaica Series, Volume 11, New Haven: Yale University Press, 1956]

The 11 day difference between the 'full' 365 day solar calendar becomes six when the five epagomenes are subtracted.