John,
I wish to underline the important fact that this Indian barrel is probably earlier than I thought at first, which is really great!
The standing integral backsight, the riveted pan cover and the overall length made me think so. They all closely relate to early-16th c. Italian and German arquebus barrels which were shipped to India and widely copied there for hundreds of years.
As far as I can tell, from at leat the 17th c., these barrels considerably extended in length, and by at least mid-18th c. lost their pan covers, with no trace of a possibly former rivet.
This might date your barrel to the mid to late 16th c., backing up at the same time 'Nando's briliant observation that the swivel is significantly different, and consequently not originally belonging!
After all I must say I have never seen such an early Indian barrel as yours!
Best,
Michael