Scooter’s first guess was right. It is for balancing the front and back radio speakers! I have the records: the original owner had the dealer, Stahl Motors in Monterey, install a Blaupunkt radio in 1970. I just don’t play it very much; I didn’t even realize that grill in front was a speaker. I would not have wanted to drill that hole in the dash!

The dumb part is that this has happened to me before. I had a 1971 Volvo 164 which had a 3 point toggle switch on the dash (we called it the NO NO switch because it was upside down and said no instead of on). Anyway, it appeared to do nothing. The funky rear speakers weren’t screwed down, so they slid all over and I assumed that was why sometimes they didn’t work (instead of me turning the switch to NO). I ran into the guy I bought it from and he set me straight after he stopped laughing.

Anyway thanks everyone.