I  had never noticed the difference in color of the knobs on the ART-13.
Kinda reminds me of this puzzle in the local newspaper – “Find 6 differences in the drawings”

and the tendency is to overlook what’s “not important”.

So why the red knob skirts and some black ?

Photo via Julyah, pres of the PSARA antique radios club.

-Hue Miller