What was the so-called “Yellow Boy?” It was the Model 1866 Winchester, so-called because of its brass receiver. It replaced the Model 1860 Henry and in turn was replaced by the legendary Winchester Model 1873. Each one was a big improvement over the previous. The Yellow boy (66) had a side-load gate at the receiver and a forestock on the barrel.
The advent of the center-fire cartridges would also allow for more powerful bullets. The legendary Winchester ‘73 was the first truly practical center-fire repeating rifle.