I think the knobs or bumps are supposed to represent light-bulbs. There was an effort in the US (post-WW II?) to standardise the shapes of cockpit controls and switches. Hence: the gear lever with a little wheel on it; the flap lever shaped like a flap; engine controls with standard shapes/colours (straight black throttles, blue wavy propellers and red spiky mixtures).
The Brits took a while to catch on. Typical controls included 'plungers' or push buttons for the gear and plain knobs or switches for the flaps.