Probably stamped in at the factory
I have a lot of dimples in my 66 Chevelle
There are also a lot in a 73 Chevelle
And many of those are there for locating a specific harness or attaching a specific accessory.
Most all of them have a purpose----just that none of those purposes apply to you.
point----had 4 dimples in the firewall of my Chevelle--3 in a triangle and one in the middle of the triangle----used for the throttle cable on a Pontiac Lemans.
also have 4 little dimples in a square for mounting a Pontiac voltage regulator.
Chevelle regulator is on the core support----throttle is controlled by linkage instead of a cable