Can I make a suggestion? Ask the person that you just bought the boat from?
Trying to find an image through Google, but not having much luck, yet.
But, here's my thought/guess: I agree - portside, so nothing to do with rigging. I could be wrong, but I don't think I've ever seen motor well lights from Grady mounted on the sides - they're usually centered, correct? I wouldn't think it's an overflow since there would be scuppers at the bottom of the motorwell through the transom? If there are no screw holes, then it won't be something like a washdown since they always attach with screws/bolts. Without holes, it needs to be attached from the back - such as the way gauges and many stereo remotes are attached. Or... it could of been a project the PO started and never finished and all it is is a hole.