Actualy the firm of Manton and Co was established in Calcutta by John Augustus and Fredrick Manton and was aquired by WR Wallis who ran the buisness into the 20thC. There was also the firms of John Manton and that of his brother Joseph who was gunmaker to the East India Co. ,thus an Indian connection. It is not clear from my listing and there does not appear to to be any direct connection between Manton of Calcutta and the Mantons of London .Unless they were sons of John Manton of London.
I would hazzard a guess that the gun you have is one that was made by or for the Indian firm .
