Cool machine! Congratulations. I've never seen the knob for locking stitch length look exactly like this before, but that's what it is, I think. You partially unscrew it to move or reset the stitch length and then screw it down again to hold it.
I have a similar feed dog adjustment on one of my Model 15 clones, except with only three positions.
I wonder what this machine was made/marketed for? Probably those additions were to increase flexibility for dressmaking with various fabric thicknesses, I'm guessing.