I will be the first to admit that an S/42 is not my main interest (made after 1918 etc.) but just from a general Luger observation standpoint, I tend to side with lugerholsterrepair. It looks like a very pristine original Luger and an excellent find. The left grip exhibits a bit more figure that you usually encounter, but not unheard of. I would want to remove the grips and verify they are original and matching.
The wear pattern is consistent with a little used Luger, probably one issued to a "desk jocky" and only carried for ceremonies.
The gun has been fired, but very little. Possibly only test fired. On a "fresh" Luger there are only two points of toggle/frame contact (see arrows in photo). It is only after extensive firing when these points have been peened down that a broader "toggle slap" wear pattern will show up.
There is light holster wear on the forward frame rail, but once again very little, indicating not much actual holster time.
Wouldn't be the first time I was wrong, and it for sure won't be the last, but from a finish standpoint I think it probably is an honest piece. As far as the "u" suffix being incorrect, I leave that to the experts.