I was trying to decide between a White 31 and an FR. I ended up getting the 31 because the FR had been sold. One question I have about this machine: in every picture I have seen, the top of the presser foot bar is rounded and matches the top of the needle bar. On my machine, the top of the presser foot bar is flat and has a tab which can be pressed to relieve or increase spring tension on the foot. What is this feature called?