Joe Manton was long gone by the time sidelock (assume you are speaking of a hammerless gun) guns came along. The odds are long that your gun was made in the trade at Birmingham and retailed with the Manton name - "Manton" held brand name value for many years. Photos would be very helpful in further identification. I'm not aware of the records of the Mantons, either of them, existing. However, we may be able to give you something of a trail with some id of the gun.