The very first Model A I looked at to buy in 1966 (a 1931 Coupe) in answer to my classified ad in a local newspaper had a similar black, splotchy pattern over the instrument panel's ribs. The older person selling the car explained that traveling salesmen in the day would strike matches over the ribs to light their smokes. I was skeptical (at the age of 17 already!) and thought he was just trying to explain away a weak selling point in his car. I was told the same thing many times later over the years, so I guess the old codger had been right back in '66. 'Still didn't buy the car, though. I ended up with a different 1931 Coupe as my first Model A, whose instrument panel had been spraypainted silver - probably to hide the black, splotchy pattern on the panel's face.
Marshall