That story is so unpretentious that it has the ring of truth to it. Many G.I.'s would have embellished the story to say that they took it directly off a German officer or some such variation. I can't imagine that anyone would make up a story as to how they found a P.38 rig while answering the call of nature. Great story.
To add to what Mikey said, early Walther p.38's used a 480 number code with year date. Beginning in late 1941 they switched to the ac code with year date.
Mauser started making P.38's in late 1942 or early 1943 and used the byf code with year date. In 1945, just before the end of the war, they switched to the svw code with year date. When the Germans surrendered in May 1945 the French took over the Mauser plant and made P.38's for the French police. These guns are marked either svw 45 or svw 46, but they have a 5-point star on the right side of the slide instead of Nazi proofs.
Spreewerk P.38's used the cyq code, but Spreewerk P.38's didn't have the year of manufacture on them. They can be dated by using a production chart, however.
-Lee