G-Man, is the bass body by any chance one of those odd rectangular jobbies?
It could be for the strap pivot - I can't remember my photobucket account details to post a pic, but the first picture in the second post here is what I'm thinking of:
https://www.talkbass.com/threads/hohner-b2b-headless-4-string.291667/
If I'm not mistaken the point of the pivot is to extend the upper horn for balance when standing and playing (there should be a strap button on the end of the plate), and to (ahem) pivot 180 degrees to form a lower bout so that the bass doesn't slide off your knee when you're seated.
Or I could be completely wrong, in which case I'll retreat to my self-imposed exile