Help with knob on TL1900

[ghw]

Can anyone tell me what use and/or various positions are for the knob located below and in front of the the seat of my TL1900? It's a black knob about 3" in diameter that can be rotated cw or ccw. Thanks in advance.

 

[JJPWAus]

Ghw,
Just a guess , is it for adjusting the speed of your three point linkage arms? I know my Iseki has a knob to adjust this.

 

[RobertBrown]

That would be my guess