The date code and 8000ish 'm' suffix serial number places it with a production date of December 1943.
The grips may not be original to the pistol, but as 2nd variation Walther grips they are correct for the pistol.
As noted by someone above, the holster is most likely a post war police holster and the value of those holsters isn't as significant as a WWII holster. But they are still worth $25-$30 on-line by the time you pay shipping.
The P1 magazine also lacks collector appeal and they can be found for $15-$20 on-line...but it's still worth $15-$20 plus shipping.
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There is a lot of snobbery afoot when it comes to collectible arms and collectors always turn up their noses at guns in only fair or good condition when there are very good to excellent condition guns still commonly available.
That said I recall Garands, 1903s, M98s, SMLEs, 1911, etc that people turned their noses up in the 1990s that bring 5-6 times as much now as they did then. That's not a bad return on a 25 year investment.
I also see a lot of people who decline to buy firearms that are longer made on the basis that the price is $50 to $100 "too high". More often than not however, those folks 1) don't buy many firearms and 2) end up paying just as much for a similar firearm in similar condition a few years later, and they just end up losing access to an interesting firearm for those years.
You almost never pay too much for a no longer made firearm, you just sometimes buy too early and have a wait a few years to break even on the deal.
These same collectors almost always ignore the value of an interesting "shooter" firearm.
In this case, the reality is that a non matching matching number AC coded P.38 in that condition will bring $500 on GB based on recent completed sales, as it has that value solely as a shooter. Matching numbers are a plus, but in that condition won't add much value. Add the police holster and P1 magazine and its value is a solid $550.
$400 is a deal, now and it'll be a really good deal 10 years from now when you either still have it, or kick yourself for not buying it, since by that time a pistol in similar condition will probably cost you $800-$1000.
There's some intrinsic value in shooting a P.38 just for the experience, and like any pistol with a history it always makes you wonder where its been and what it would say if it could talk.
It has some freckling but little if any pitting, so it's also a good candidate for restoration. It won't reduce the value as a shooter or as a collector (given its present condition), so all you'll be out is the cost of getting it properly refinished. What you'd have then for that additional investment is a nice looking, good shooting AC coded (Walther) P.38.