When I look thru Rifle of Colonial AMerica, I see some guns that have wood behind the fence, and others don't. It's not a deal breaker for me, and I certainly wouldn't glue wood in to fill that gap. You may choose to do otherwise, especially if you're making a bench copy, where the original has wood in this area.
I also see that some guns have the fence lined up with the breech of the barrel, and other guns have the fence forward of the breech. In short, this is a function of how long the breechplug is.
I hope this helps.
Tom