A classic for its era

That engine's by no means unusual for late 1840's/early 1850's standards. The inside motion, short driving and lead truck wheelbase, skinny rear dome and other features are typical of the period. Some of the styling like the cab are more representative of post civil war practice and probably reflect an overhaul. I'm somewhat surprised it doesn't have a bury-style boiler; perhaps that was altered in the rebuild? What most folks think of as a "generic" American-standard 4-4-0 type didn't really take on its well-known mature form until about the mid/late 1850's and 1860's.

The inside motion complicates maintenance but also permitted smoother running with less side-to-side wobbling. Early lead trucks didn't dampen such motion as well as later designs.

One of the problems with steam locomotives that persisted throughout the entire steam era was how the engines themselves tended to last well into technical obsolescence. By 1870 an 1840's-vintage engine would've been bordering on obsolete yet often have much life left in its machinery. Such engines often ended up sold to industrial or short line roads or found themselves in secondary work, transfer service, switching, etc.