Eger, you didn't cause any controversy, open and free discussion about the OP's rifle is needed to shed light on it. I have no illusions that the P.38's U marking have a direct connection to the subject K43, and nobody else should either. I also don't think the location of assembly (Walther proper or Nuengamme) is of much concern to the rifle, as both were run by the same company.
In my opinion the "U" markings used to cancel waffenamt controlled parts is also not something you can say is uniform to the P.38 - the Spreewerk factory could have used the U in a different manner, meaning they used it to approve the pistols (and parts) outside of the Waffenamt control for their own reasons.
In my opinion the "U" markings used to cancel waffenamt controlled parts is also not something you can say is uniform to the P.38 - the Spreewerk factory could have used the U in a different manner, meaning they used it to approve the pistols (and parts) outside of the Waffenamt control for their own reasons.