What's This Button Do?S

I was just in a very interesting car for an upcoming Jalopnik Classic review, and I noticed this control on the dash. Now, in a classic car, you often encounter symbols that are a bit different from the standard set almost every car uses now. I quickly flipped through my Mental Dash Control Symbols chart and came up blank.

So that the hell does this do? Based on the shape and feel, it could be either a knob or button. Maybe it's for driver hypnotizing? Auto-spin-out feature? Time warp tunnel opener? Coriolis Effect tester?

I eventually learned what it's for, so there is a correct answer. And I'm sure someone will recognize the car pretty soon as well. But first, let's get in Kinja and do some speculating!

UPDATE! I think one of the commenters, ejp, nailed it. It's the instrument brightness control for a '74 Alfa GTV. I don't really get what they were going for, either.