Odd Firewall Holes

[29 Tudor]

I have a '29 Tudor that has a manifold heater on it along with the attending hole in the firewall. However, it also has an array of holes to the outside of the firewall (the holes with the string going through them) that I am puzzled about. I wonder if it had a fan forced heater at one time and these holes were for return air to the heater.

Any ideas what the array of holes was for?

Thanks in advance,

Steve