Looks like a former owner's add-on. Not sure what it does? Is that an extra piece of wood on the floor by the screws? Where does that piece go? It appears to be a different type of wood than the rest.
There are no springs or glider bars on this chair...this is the rocker mechanism. If you go back the shank of the big brass screw in the middle goes up into the front part of the heart...forward it goes into the back part. Ease of motion and stop all rolled into one. No more broken springs or glider mech. the piece of wood at the bottom covers the mech, and the brass screw goes through it into the chair seat. It does look like an add on...but there are no screw holes or other marks under the chair where springs or other had been. It is a different type of wood...fine grained...probably a box wood.
