"The replacement of ṣ and s by ś is one of the main characteristics of the Magadha Prakrit, as warranted by Vararuci's rule, ṣasau śah. But in the Kamarupa inscriptions, we find the reverse of it, i.e the replacement of ś by s as in the word suhańkara, substituted for the Sanskrit śubhańkara in line 32 of the Subhankarpataka grant of Dharmapala." This contrary rule was first pointed out by Dimbeswar Neog (Sharma 1978, p. 0.25), (link) Sharma, Mukunda Madhava (1978). Inscriptions of Ancient Assam. Guwahati, Assam: Gauhati University.