I reviewed the guard in all pictures where it is visible, and it appears to be original to the gun to me. Its front extension fits the front mortise well, and the rear extension butts against the extended toe plate. Stocks invariable shrink over the years (a good sign, since it verifies the stock is old and original), popping some inlaid metal slightly. At times the edge of a patchbox, or large inlay, will raise above the wood surface due to wood shrinkage beneath it. Any slight rise in the end of the guard's rear extension is normal for an old rifle. I see nothing suspicious with the guard, either in its fit, or in its style, engraving or finishing.
If the concern is that the back end of the trigger plate has a gap between it and the guard's rear extenison, again, that's a normal occurance on some (a good percentage) old rifles. I think you have a nice, original percussion rifle, probably made a little later than one might think at first glance (1850s), based on the forestock inlays being mounted with small screws and the barrel not being longer. Shelby Gallien