I'll have to dig a little on that at the C body sites. If it weren't available on an a-body, I'm going to guess it was retrofitted from a Chrysler or a high end fury or Monaco or Polara or the like.
It's for the "submarine mode" function, which by the looks of that dash didn't work so well. Has that car been in the bottom of a lake for the last 30 years?
The car belongs to my neighbor. Its pretty rough but the price was right. 360/727/8 3/4. Good floors and no major wrecks. I say pressure wash it and mini tub it.
Good looking car, probably worth a pile in parts alone. Is it a fastback or a notch? Digging hard on scalloped paint scheme, I'm guessing last on the road in the mid to late eighties, maybe?