Over the years I have seen three unique heaters for Model A”$ that had floor vents ( but none of the three was actually installed on a car - pieces of one came with a station wagon project that is on my “to do” list). This type of system was attached over the muffler area and I believe also had a front floor outlet - each outlet had short ducts between the muffler area and the floor. I am prone to believe that the piece in question would appear to be more in line with a deflector used on some of the earlier Model T’s, but since it would also seem that the holes also align with the front floor vent, I can only speculate. The knob in question would have probably been used to open and close the vent.