That is a 521 flasher knob. Look in this pic and the 2nd to last is the 520 style. At some point they started using the red light symbol for the emergency flasher, maybe they used it first and the went to the black style that we are used to. I would like to look in some 520 owners manuals but every time I bid on one I get outbid.
More reference here is the light switch difference
heater fan difference
wiper difference