Seriously - I believe it it as I describe above , there's probably a special name for those characters ending a paragraph. You all most have seen them in booksIt's just to fill the space and make it look complete
The designer probably tried leaving it blank and then, after trying lots of things, thought 'Ah yes what we need is a - O -'
and his boss then said unto him , Cool - send it to production buddy boy and lets go to the pub
- O -
My guess is that it is an alignment mark used during manufacturing - optical sensor in the assembly machine ID's the mark as a reference and thus assures proper alignment of component fit.
Of course, I have no backup or "proof" that is the case, but it makes engineering sense - to me, anyway. ;-)