Or possibly 3 sections; obviously the thicker at the breech which is why the pattern if often slightly different
Hard to tell but the arrow may be the weld line between the sections
The other arrow is likely an area of decarburization from excessive heating during the helical hammer welding of the tube
The neat rib pattern is from the same laminated steel rod turned on its side and ground to fit
https://docs.google.com/document/d/1...Ix2xJM9bA/edit