Denis,
I understood your question a little differently - are you talking about the piece of wood behind the fence to which Acer and Roger are refering, or are you talking about what appears to be a very minute gap at the upper portion of the fence where it butts up against the barrel? If you're talking about the wood, then Acer and Roger have you covered. If you're talking about the metal to metal gap then you have a couple of options, as I see it. First, since you said the bolster is tight and the fence gap is too small for powder to get in, you could ignore it. It may provide a crevice for fouling, but that could be addressed by simply pulling the lock when you clean the barrel (something I routinely do, but others have different opinions). Another option might be to peen the fence on the back side to fill the gap. The gap appears very small, and it probably wouldn't take too much to get the metal to move. Hope this helps.
Eric