Tough call.
Simple research shows the Mm Maslin dated late 1820's, with several available for review.
While the the lock, barrel and hardware of yours shows appropriate age, the pristine condition of the stock and lack of patch box indicates it may well have been or is a nicely done re-stock of the available components.
Re-stocking is an expensive thing to have done, but the result usually doesn't command the same value as that of an original.
Do you have any documentation?,, Verbal History? Or knowledge of the back ground?