Actually, that's not an original magazine; there were no blue magazines, and none marked as you note (at least not through 1980).

With that ser.# the slide is most likely welded also. As Conley notes it's probably not the best idea to consider it a shooter in that case. You can almost certainly tell from the outside, looking just where the scallop under the muzzle end drops down to the lower edge at the side. If you can't tell for sure there you can check the inside of the slide...