Yes it is a very small light bulb, and activated when the high beams are on.
I always thought that the 170S and 220 were the same switch with different knobs. But you are correct, the 170S and 220 are different on the knob side because these models have a pull out feature for engaging the fog lights in the first position, and do not have the internal high beam indicator bulb.
Carl, it appears that my extra switch wouldn't work for your 170S .... sorry.