How do you know if it is original? Guns of that period would almost certainly have had heel plates screwed on ,they may have been glued as well with a bone glue but screws would be the main thing . If you want to remove it try heating with a hot air gun as this will soften some types , others are on for good and you can damage the wood in trying . If this is the case then a good rasp is my favourite . You also do not know what is behind it so yo may find a hollowed stock ,lead weighting oetc .