About the only real difference between the post-war P-38 and P1 is the stamp on the slide.
The internal parts are all the same, and most frames, slides, and barrels interchange no matter whether they have the reinforced slide or not.
Most parts will also interchange with the P4.
In fact, many WWII parts can interchange.
The situation with the P-38 and P1 is similar to the USGI Model 1911 pistols and the Colt Government Model.
The actual guns are the same, just that one was made for the government, the other was a commercial model.