In a way, you pretty much summed up what it is, perhaps without knowing it. First, yes it has the 4 digits of the carbine's serial number stamped into the traditional slingwell location, although it's one of the most uneven stamps I've ever seen and I've seen a lot of Bavarian carbines too. Second, the stock appears to be a BR-B (Milton Bradley Co.).
As far as the cartouche goes, much of the details one needs to see are not there. What I'd do is simply measure the thing and you can tell if it's Winchester or IBM, which it most likely is. BTW, the later style Winchester cartouche is about 1/2 an inch tall while the will come in closer to 3/8 of an inch. Finally, the stock has some of the yellower tones of a Birch. Hope my ramblings help!