For durability the later version as stated with the strengthening hex cross bolt (visible in the pics above), The later versions also have thicker frame /slide rails as well as white lines on the sites that make aiming easier.

That being said the earlier civilian imports ( Interarms ?) seem to bring more than the X German PD guns imported by Century,
Btw... always check to see that the upper and lower serials match as many were mix matched parts guns.

Another interesting tidbit is the early post war aluminum frame guns are marked P-38 where later guns are marked P-1, interestingly the one in the center pic above says P-38 but is a 1976 gun so very very late for a P-38 marking IIRC.

The weak link in the P-38 (as well as S&W 39-2 / 59 series) is in the safety system which can be inertia fired if dropped.

Also well worn versions have been known to overcome the decocker so that in rare instances decocking the gun on a live round can fire it.
This was changed in the next evolution of its design Walther P-4 and Walther P-5.