Yes, just think of device on the left barrel as an elongated nipple. In fact many of the original makers of this era, simply made up an elongated nipple for the under barrel. This gun is a wonderful piece. I made two bench copies of the a Solomon Heriger rifle owned by his grandson Russell Harriger, author of the book Longrifles of Elk, Jefferson and Clarion counties. This gun shares many of the same inlays as the gun I copied. Kelly Harriger G.G. grand son of Solomon owns several Harriger rifles. These guns also bear evidence that Solomon used inlay patterns over and over.
As an aside the Jefferson Co. Longrifles Muzzloading Club meets & shoots in Harriger Hollow near Brookville, PA. The hollow is the exact location of Solomon's gunmaking shop. The property has been provided for use as a Muzzloading club by the Harriger family and was the site of the first Eastern Primitive Rondy.