Hello there. What a lovely musket. Even with the grunge of ages upon it.
Based on the photos you have provided, this is likely a Pattern 1839 musket, but, it could also be a P/1845 Extra Service musket. To help...
I assume it is .75 calibre? Can you confirm that the barrel is held in place in the stock by barrel key/wedges rather than pins? (looks like keys based on the photo, but...) Is there any sign of there having been a rear sight on the barrel at some point? If the proof/acceptance marks on the barrel are still visible, can you share a photo of those, or include them in a reply. Any markings on the buttstock?