Re: Mystery hole
Not disputing Bondo, but they also were sometimes used to run fuel, and battery lines, from the fuel tanks, and batteries below the splashwell, to the motor.
They seemed to be very common in the 70s. This is my 1974. It just had a round piece of trim, whereas, yours was square.
In fact, similar things are still used. See this 2008 Alumacraft Pro 175. The fuel line, control line, and battery cables each go through the transom. Course, it is a tighter, custom fit, but back in the 70s everybody used universal fuel lines, and battery cables, so the hole had to be universal.