15.7.18.23:45: WHY DOES TENASSERIM END IN M?

I wonder what the etymology of the name Tenasserim is. Seri looks like Sanskrit Śrī, but what is the first half? Is it Mon?

None of the major languages of the area have e or m in their versions of the name:

Burmese: တနင်္သာရီ <tanaṅsārī> [tənɪ̀ɴθàjì]

Mon: တနၚ်သြဳ <tanaṅsrī>

Thai: ตะนาวศรี <taḥnāvśrī> [tanaːwsǐː]

Malay: تانه ساري <tānh sāry> Tanah Sari

I think e may reflect a schwa and an epethentic vowel between s and r. But why does the name have a final -m in Western languages?

Why does the second syllable have different codas (nasal, [w], or [h])?