Prior to 1877, the trapdoor breechblock had a high arch machined out of it, making it lighter.
The Model 1877 no longer had the arch machined out of it, though the reason for this modification is not clear.
Gas ports along the upper ridges of the receiver were also extended, which may explain why the arch on the breechblock was removed at the same time.
After the Battle of Little Bighorn, Springfield Armory was assigned the task of providing a cleaning rod for the carbine version that could also be used to remove stuck cartridges.
This required a new butt plate specifically for the carbine, as the longer rifle already included a cleaning rod and did not need these modifications.