Sorry, that's not correct.
The P-38 was made after WWII from 1956 to end of production in the 1990's. The post-war models had aluminum frames, with a few made with steel frames near the end of production as limited production models.
The P1 was just the P-38 as made for the West German military and some very limited commercial sales.
The P-38 was made for German police, foreign military sales, and for commercial sales all over the world.
Currently, Earl's Repair Service is the Walther repair and sales rep in the US and they still sell commercial aluminum and the limited production steel P-38 pistols.
So, Post-War there were BOTH P-38 and P1 marked Walther pistols made and sold with aluminum frames with a tiny few P-38's in steel.