Below will help you understand what "copy" is referring to. Part of my Fathers collection. This has a big dent on one side and a small what I thought was a hole on the seam but I put water in it and it did not leak. Honestly most of these do have dents because of the nature of the material. Read below - this is an awesome piece of history.
The M-1858 canteen was a standard issue for Union soldiers during the Civil War, and they likely had a sufficient supply. The Confederate States of America faced significant resource shortages, including manufacturing capacity and access to materials needed for standardized military equipment. Faced with these limitations, the Confederacy would often copy or adapt Union equipment, including canteens. The term "Confederate copy" would have been used to distinguish these canteens from the standard issue M-1858 canteen. The "tin spout" likely refers to the design of the canteen's spout, possibly made of tin, which was a common material during that era.