It's probably an aftermarket handle like the one I have on my Bridgeport. It allows the handle to be reclocked so you can always get the best leverage. I usually start mine at the 11:00 position, pull it down to about 7:00 then reclock it for the next pull. It also comes in handy when the tool lengths are different. You can lower the quill to the point the tool touches the work piece then reclock the handle to a comfortable position.
The handle on mine is spring loaded with a detent that slips into a hole in a plate attached to the pinion just like in your picture. To reclock it you pull the handle away from the head and rotate. When in the desired position you release the handle, and the detent goes into the nearest slot. Mine came from H&W over 20 years ago.
Here's a picture of it in the 2:00 position, just to keep it out of the way: