Rabbeinu Bachya (author of Duties of the Heart, 11th century)
and Rabbi Moshe Cordovero, the Ramak (Pardes Rimonim,
16th century) were two of history's greatest Kabbalists, and each hinted about equidistant code sequences in the Torah (fascinating biographies here and here).
This page from Professor Haralick presents the first such mention, with an extremely accurate derivation of the length of the lunar month arising from it.
The Ramak discussed several methods of extracting secrets from the Torah, equidistant sequences being one of them.