I picked up a couple small planes recently, both in need of good cleaning and tune ups. One is a Stanley 110, and then this one here, not sure of mfr but is marked Made in USA. So my question concerns this little knob in the front holding down a little lever that doesn't seem to do anything shown in last pic. Curious if anyone knows what it's for or perhaps that some original other parts are missing that would make it make sense. Thanks!