Dating an inscription in India, where most carry no date, is done by comparing the shapes of common letters against inscriptions whose date is fixed by a named ruler or a regnal year. The method needs letters that are frequent enough to appear in every specimen and distinct enough that a worn example can still be identified, and this letter qualifies on both counts. It appears in quantity in the Ashokan edicts, in the donative records at Sanchi and Bharhut, in the western cave inscriptions and in every later hand, so its development can be traced continuously rather than reconstructed across gaps.
What that tracing shows is a script diverging steadily after the Mauryan period. Ashokan Brahmi is remarkably uniform across an empire that ran from the northwest to the far south, which is itself evidence of a chancery imposing a standard rather than a script evolving locally. Within a few centuries the uniformity is gone, and regional hands develop that a specialist can tell apart at a glance.
By the Gupta period the northern and southern lines are clearly separate, and from there the family fans out: Siddham and the Nagari group in the north, the Sharada line in the northwest, Grantha and the Tamil, Kannada, Telugu and Sinhala alphabets in the south, and the Pallava chancery script that carried the system to Southeast Asia. This letter can be followed down every one of those paths.