Guys are always quick to say "factory mismatch", but the factory didn't mismatch serial numbers, especially at that early date. These were Germans, after all. Something's right, or it isn't. Alles in ordnung.
I have a dou 42 98K. My uncle took it from a pile of surrendered weapons shortly after the surrender of the German garrison at St. Nazaire in 1945. Everything matches except the bolt, which is also dou but just a few serial numbers off. He took it off of a pile of bolts right next to the rifles. I figure that the unit that surrendered the rifle was equipped with 98Ks straight from the factory at some point, and there were sequentially numbered guns present through the units history.
As far as finding that matching mag goes, I wish you good luck. To say that the odds are against finding it is a real understatement. I've been looking for my 98K's matching bolt for over 50 years. I came very close once, but it wasn't quite a match, having been made by Mauser. I don't think that your magazine being one digit off detracts an in any way.
Nice find!