I don't remember where I got the 1943 date. That may have been what was given in my reprint source, which I believe was DC's original version of "The Greatest Superman Stories Ever Told."
It looks like DC isn't even consistent on this. The "Superman in the 1940s" TPB (which also reprints the story) gives the date as February 1940.
I bought the original two pages that were for sale at
Jason's link. Hopefully the date will be on one of them, although it doesn't look it. Maybe on one of the flip-sides.
I'm wondering if there are any clues in the story itself.
According to Wikipedia, the term "Westwall" was widely known in 1939, so that's no use. It also states that there were no battles there until 1944, so that fight scene in the story seems like it's a bit misplaced, regardless of whether it takes place in 1940 or 1943.
Stalin could be a clue - Germany and Russia were allies until 1941, when Germany turned on Russia and invaded. At that point, Stalin took direct control of his country's armed forces. Germany and Russia could still be allies in this story, but that also looks like a military uniform on Stalin.
At this point, I suspect that the story is actually from 1940 and that the 1943 date originated from a mistake in DC's first reprint credits.