It's definitely not a hole, it's a plate or a cut pickguard. He's resting his thumb on it in the second photo.
I'd say it's a hole that's covered by a plate. You can see the screws in some of the close-up photos. There's a similar cover on the bridge J pickup, which makes me think there were some soapbars in there at one point, then changed over to a P/J setup. I'm sure the plates are there to cover the unfilled holes from the reroutes. Just a guess, but I've done this a few times, so an educated guess.
That’s not the one that I saw. But I’m glad to see this!!! Yes , that is his red MusicMan StingRay. So we know that Fred was playing that bass in 1986 according to video description. Thank you !!!