Brent, that is the barrel selector.
On these higher graded Smith's above a No.3 grade, the checkering is done by what might be called a master checker. The engraving was done by Albert Kraus as he only did the engraving on the higher graded guns.
When some of the higher grades with gold, the case hardeners had to be careful not to melt the gold out of the engravings. It has happened and wasn't a pretty site I was told.

David