Chain-drive feed systems were used on the Advance, Accentra-Cast, and the XXV prior to the UL Feeder bodies which allowed for a direct-drive auger motor. This sprocket is mounted on the auger motor which is pictured on the left in the picture below.

The early versions of this feed system used a smaller primary gear (motor sprocket) and a larger secondary gear (auger) which also used a shorter chain.Asof2013, that systemwas updated to accommodate a1:1 gear ratio using two 3 1/2" sprockets. If you have the old version, you need to update your stove as all of the older parts are obsolete.