Help with identifying 3rd setting on ruffler foot

Hello!

I recently purchased a bunch of antique/vintage feet for my model 66 sewing machine. I really was excited about the ruffler foot in particular.

It’s branded Greist USA and nothing else. I was able to get it on my machine and it looks to be working (haven’t sewn with it yet as I need to finish my current project) when put through the motions with no fabric.

I wanted to find what all the adjustments do and while I know the ruffle/stitch one obviously I’m having a hard time figuring out what this specific screw does.

Looking up others online they either don’t have this adjustment and it’s just a rivet or people do have it and never mention it ever in the videos or articles.

The knob should be how “much” ruffle you get per ruffle, but the screw looks to effectively change how long the arm is doing the ruffle. In theory this would also adjust how much ruffle you get?

Does anyone know what this is for specifically?

My leading theory is that the three settings are \- ruffles/stitch \- ruffle fullness \- ruffle depth?