Halanta words changing into ajanta words during borrowing is typically a feature of South Indian languages. That ending vowel being u is typically a Telugu feature. 

Samvat is pronounced as close to Samvatu with the lat vowel being nearlly u in Telugu and some other south Indian languages. 

But Samvatu changing into Sammati is uncommon in educated pronunciation. But possible in the case of a scribe being rustic. 

Possible in a Telugu inscription or manuscript.

On Thu, Jul 2, 2020, 6:38 PM Martin Gansten via INDOLOGY <indology@list.indology.info> wrote:
I just encountered the word saṃmatu at the end of a (Sanskrit Devanagari) manuscript where I would have expected saṃvat. I haven't seen it before and wonder if colleagues have -- perhaps it is a NIA form?

Best wishes,
Martin Gansten

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