Part 120 is a small handwheel that when done up locks the worm gear to the spindle quill feed which is then driven by the fine feed handwheel worm at the front. When it is loose the quill "drilling" handles can rotate freely and they drive the quill.
See "Fine feed locking knob" on the SX2.7

Edited By JasonB on 03/01/2018 15:22:43