1. In Telugu (probably in all Dravidian) inscriptions, anusvāra , a full circle,  was a later borrowing from Northern Brahmi orthography. 

2. There is ardha (half) anusvāra, a verticle half of a circle also in classical Telugu orthography.  There is evidence to believe that this script symbol was used to indicate nasalization of the preceding vowel.

There is evidence also to believe that the script symbol survived even after the nasalization was lost in pronunciation by the preceding vowel.

3. The ardha (half) anusvara, the verticle half of a circle is a later invention in the Telugu orthography. During early Telugu inscriptions, when the ardha (half) anusvara, the verticle half of a circle was not invented yet, full anusvāra, full circle itself was used in the place of ardha anusvāra also. A full circle not followed by a glyph for a nasal indicated the ardha (half) anusvara (nasalization of the preceding vowel) whereas a full circle followed by a nasal indicated full anusvāra (nasal first part of a cluster).

This is just to see if this provides any clue here.

Nagaraj   

      

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