Reasoning behind the type 1 & 2 Buffalo nickel
The designer, James E. Fraser designed the niclel with the buffalo standing on a mound of earth.(thus the type 1 or first style). The coin didn't wear very well and the design was changed half way through the first year of production (1913) and the buffalo was lowered to a flat level piece of land.(now called the type 2 or second style).
This was the design used from that point forward and no more mention was made of it for the following years of production.
But, they all had E. Pluribus Unum on them above the buffalo's back.
My guess is that it is a button.