My guess is by the factory. It seems that the aftermaket stuff has the front sight on top of the muzzle instead of the slide. And between you and me, I like the aftermarket design better. Looks sharper, more elegant. A classy fake.
Well, after thinking about this product, I decided to do some research. Both Kersten's tome on Walther and Gangarosa's two books on Walther and the P38 show that the P38k was developed concurrently with the P4 in the early seventies. As such, a REAL P38k will have the P4-style decocking lever only (not a safety), shortened hammer spur, adjustable rear sights, and hexagonal steel reinforcing pin in the frame.
Those pictured on MGS's website have none of those features, so they must be after-market copies.
And if you read their write-up carefully, they do state that they are cut-down P1's, and not real P38k's.