All I can say for sure is that my '78 doesn't have one, and that the top of the CB trunk is definitely below-the-waterline. So yes, potential catastrophic leaker.
What type of bung is it? Wine-bottle cork? Levered bung? Screw type (like in the live well of a fishing boat)?
I can't imagine why someone would drill a hole here. It'd be lousy for drainage, as it's several inches above the bottom of the bilge.