No one knows with certainty- of course- but the topic has been well discussed over the years, and I think there is a good possibility the two variations of the markings are related.
There are two different BS marking styles; one is block letter "B.S." and its marked “usually” on top of the receiver, in a similar fashion as the "Zn" marking. The block B.S. is not accompanied by a date though. This marking can also be found on many things- unit marking discs and buttstock combs (rifles), pistol magazines and accessories. And when it is, sometimes it’s followed by numbers, as though it were a unit marking.
The intertwined "BS" is, much more problematic. It seems at first glance to be an organizational marking, located “usually” on the barrel shoulder or buttstock. It can be found on commercial pistols, machine pistols, and all manner of rifles, in all sorts of locations.
The problem is this intertwined “BS” is also found in the same location on top- and side of the receiver. I have several examples documented and they are identical to the block “B.S” examples, except in the style of the BS.
The foremost authority- imo- on such a topic is MarkW, and even he seems to be confounded on the topic. He once told me in an email that if the marking were on the receiver it probably represents work done at the depot at Berlin-Spandau, but if elsewhere its an organization marking. Very possibly Bahnschutz related.
Problem is, of course, is that both styles are seen on the receiver, and at least in a couple cases the same markings are found elsewhere on the stock (block B.S. on a marking disc, or underside of the buttstock comb, while the intertwined “BS” is usually as yours is.. though like the EWB brand with slight variation to location)
I certainly am not confident enough to question Mark’s assessment with what little data I have collected, however I tend to think this marking (both) is more likely related to the depot at Berlin-Spandau than to the Bahnschutz. Perhaps the marking style is related to different periods, or for different purposes (different work shops- one for “rework” the other for builds or maybe superficial “acceptance”? Many of the block B.S. seem much more worked over- though not always.)
Another glaring problem is the fact most of these rifles with the intertwined “B.S”, do not seem to have gone through much repair work- as you can see this action you have here is original. The barrel/receiver is 100% Danzig, as it left the arsenal. This fairly typical of this marked variation, but several have obviously been reworked- though possibly subsequent to the “BS” marking.
Anyway, - I do not think collectors have a firm grip on the purpose or meaning of this marking. Either of them!
Can you do more of the stocks acceptance markings? Cypher etc..
By the way, Ambrose wrote about this rifle, and another Kar.98a similarly marked in the June 1992 KCN.