As Doug has said, it was only the first of this design to have the small section in the middle. This was, I assume, a carry over from the more basic carpet-only tools used on the 62 and then on early 64, which had very small section for cleaning threads.
Although coverage would have been minimal, I think for cleaning threads it was possibly adequate. I think the change was probably more to do with the overall weight of the floor tool, as this tool was very heavy and would possibly require a good deal of dexterity to keep the tool in the upturned position on the tools with the smaller section.