On some restocked rifles I have seen, if the barrel was signed by the original gunmaker, the second gunmaker re-using the older barrel often left the first gunmaker's name on the barrel... I presume out of respect for his elder, especially if the barrel had been hand-made in the original gunsmith's shop.
This Dickert rifle, while a great gun, is a bit on the plain side without butt carving and somewhat limited engraving. While speculation on my part, it seems to be the type of gun where the owner may have requested a good quality but lower cost gun, perhaps provided an older barrel from a family rifle [or Dickert had one], and was willing to re-use a good older barrel while limiting the amount of decoration added by Dickert... keeping the gun reasonable.
When I see the older looking "M.R" on the top barrel flat, where guns were normally marked by the gunmaker, it makes me wonder if the barrel might have been a good quality earlier barrel by Mathias Roeser of Lancaster, who worked in the area earlier than Dickert.
Shelby Gallien