Unlikely that's a period Joseph Manton lock, as he was sort of a pre-percussion era maker.
I've only seen flint locks marked as Mantons,though I suppose there must be conversions.
I would send an email to someone at the "track of the wolf" website (google it),and see if they can shed any light on who might have made this. I suspect it was a custom rifle built from kit parts,and Track of the Wolf is a pretty comprehensive seller of kit muzzleloader parts. Value is anybody's guess...