I have an 1863 Springfield rifle with the 1870 Allin trap door conversion on consignment. The owner wants to sell it but I have no idea where to begin price wise. I knew something was off immediately due to the shortness of the forestock and the fact that the barrel is octagonal. Am I correct in assuming the original barrel was probably damaged somehow (maybe after a resleeve?) and replaced with this barrel? Has anyone seen anything like this? Is it junk or unique? Thanks!