This coin plagues me, there does not seem to be a consensus on when Ahom(Assam) started minting their own coins. It seems cowries were mostly used and circulated along side any coins that were used at least until the 17th century ad. Not only that, but there are at least 4 possible languages, the rulers often are not mentioned on the coins, and I cannot find a decent reference for them. NGC seems to have the most listed but they are often incomplete, and they only have the one 1.5 Mohur. I found several more, they are all unique, other then being 17 g ~ and 25mm. I cannot find any mention of them in a reference, it is always an auction or a certification.
However, since they don't seem to be well documented, your may be real. I do have a couple concerns: First the lines bordering the dots seem to be offset in places, Second there seems to be some raised dots around some of the devices, and third all of the letter/numbering is sharply angular. Here are some good pictures of 1 Mohurs for comparison.
http://coinindia.com/galleries-assam.htmlI also found this description in an old auction catalog:
https:// (130915) Not Allowed Due to Copyright Issues/spinkandson/docs/16041/69
I think you need to get a XRF analysis, It may be a jewelers copy or it might be a re-struck coin, but if it is real it should be very high purity.