Yes, that's definately an 1853, but as everyone has said the rear sight is wrong. The originals were 58cal rifled but the smoothbored sepoy explaination sounds plausible.

We also gave them loads of smoothbored SMLEs in the early 20th century. They were bored to .410 and used by the Indian Police for riot control. Apparently the cartridges were loaded with 3 x .410 cal round balls.