Unless this is a factory overbore, Steve, your gun has been backbored. The single is full choke by dimensions and no doubt shoots like that too - I've never seen one that didn't. It's not a tight full choke - unless it shoots like a tight full choke, and I'll bet that instead it shoots like a loose full choke.
The O/U has been choked for doubles, and in my opinion, you should be shooting 8's for the second shot (the over barrel) and similar or smaller for the first shot. 8's, 8 1'2, 9's should all do fine. If you plan to shoot it for singles, I'd go with 8 1/2's if you have the choice. You could also shoot the top barrel for singles, after you do the following:
Since this barrel has probably been worked over, you really should go with a rest to the pattern board and at 13 yards, find out where it is shooting, since you never know.