US canteens are makers marked, Japanese canteens say Japan on them. Easier than looking at the weld. Ran into two of the Japanese ones yesterday. About half of what I find these days are Japanese. I wouldn't use the weld to ID them because some of the early ones do have large welds. You can usually tell just by picking them up. the gauge of the aluminum is heavier with US canteens.
I would say no. Most people are asking $5 for them with the carrier. I suppose if you wanted one just to show people the difference it might be worth having but I have not plans to get one.