Brien, I saw that auction too but didn't bid. I got the impression the barrel is aftermarket and has been added to the 1938 receiver. If the barrel is as good as described (the seller has excellent feedback) I think you did all right and will have a nice poor man's Navy Luger. Of course the Mauser receiver will look a bit weird with your DWM toggle, but the average joe at the range won't notice. Hey it's 9mm and should be a lot of fun swapping between 4" and 6" uppers.
Do you plan to get an ejector and sear/trigger bar so you can quickly swap uppers? I swap uppers on my mismatched DWM and both work well with my trigger/side plate but this isn't always the case. Hopefully you won't have any problem with your side plate. Also it would be good to have a gunsmith check it out just in case, especially headspacing. I did that myself before firing with the new upper.
Good luck and definitely post lots of pictures when you get it! Let us know how it goes. Don't worry, you done good...
