To add to what Vincent stated, dealing with actual observations, Kar.98a with (cool) unit markings are very rare, most encountered are MG units, Jeff Noll and Craig Brown have both owned them, both I believe are 1916's, plus there are a few others similarily marked, but very rare (real rare, like next to impossible, not just hard to find rare).
If you are looking for Sturmbataillone or Sturmkompagnies unit marked, I have never seen one, not even in books, so I would be extremely cautious or skeptical of any that showed up. If one were to come up, such a rifle would command huge attention, no other subject draws more attention than Rohr (SB #5) and any Kar.98a associated with an SB/SK would be highly sought after. I should also say that, I beleive Jeff Noll, but possibly someone else, has a really neat close combat unit marked 98a, some small organization, but i couldn't find it in my database. It was a long unit marking and quite unique, it is the only thing close to a SB/SK marked rifle I have seen and obviously quite different.