Lifting Block ("trapdoor") Conversion, probably on P53 British Musket

Please help me to identify this Unknown Rifle

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GENERALLY:   I believe that this rifle started out as a P53 British Musket, but I don't know enough about them to be sure.  Can anyone shed some light on who might have made the conversion, what exactly it is, and for whom it might have been made?  Thank you!

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Page built August 3, 2002

Updated Dec 3, 2021