Oops
Should have done my research first. Sorry group.
"Gleaner
The gleaner switch moves the bar on the rug side of the tool from smooth (off) to the grooved side (on) it works well on most types of carpet to better pick up lint, pet hair, and also just lift the pile. While this tool is still made today, the gleaner is gone and it is a smooth bar, but in the 70s it was changed to a non switch style with finer diagonal grooves. I’m not sure why they went to the completely smooth style as the finer diagonal grooves style worked on all types of carpet but wasn’t quite as effective as the original but better than nothing. The original style like yours worked on most carpet but loop type and some others it would just grab and get stuck so then you would have to switch it off. It should never be on if that side is used on any hard surface for fear of scratching it and damage to it should it be dragged on cement which shouldn’t be done anyway. It really is something that became obsolete once the power nozzle was introduced which does a far better job of removing clung on lint, dust, hair and threads and also fluffs up the nap".