Welcome to the forum!
Since the barrel does not match, it is possible that he brought it back from the war.
I would think hard about swapping the barrel out

as the 6 inch barrel is a swell one and 30 luger shoots very nice. Of course, the cost savings is true, although figure $150-$200 for uninstall, install of a new barrel and of course, cost of a 4 inch 9mm.
You could look around for an upper already in 9mm and have the best of both worlds
Ed