Best as I know, it is not a knob. It is a raised section of the side panel. As you've noticed, it is centered over the upper spool drive. On older Rolleiflexes this spool drive gear is held in place by a flat spring washer between the gear and the side panel, pressing the gear down. For reasons I don't know, Rollei changed the design for the F and later E cameras to have a coiled spring and axle setup. This required making this bump for the new additions. Yes, strange in ways, but I doubt Rollei made new stamping tooling just for the fun of it.