Well spotted, Webb. You seem to have a slightly wider frame than I do as my copy only extends to the lamp. Jimmie was probably meant to stop a foot or two further back when then set up the shot. There are a few other scenes in this episode that were obviously filmed in a studio. I can see a corner in the scenery in one shot, but no edges like you spotted.
I noticed something else in the Boar's Nest scene. When Bo sneaks into the back of Rosco's patrol car, the driver's seat is all the way back, so there's hardly room for Bo to get his legs in. We can see the seat in this position as Rosco drives off, but we can't see Bo. When we do see Bo, he's got his whole body between the front and rear seats, so the driver's seat must be about six inches forward. I'm sure Rosco would notice that much movement - I know I would!