These actually aren’t a K98k. They’re a variation of the Mauser Standard Modell, a commercial rifle that was sold primarily to international clients (China, Ethiopia, Japan etc.). This type was order by various German government and security institutions and is often referred to as the ‘Banner K’. They were ordered by the postal service, railway service, SA, and police. They’re quite rare to find in original condition. I have one in my collection thats associated with police service, possibly SA.
As to why did the postal service need them? They actually had the largest orders funny enough. But its important to remember that the postal service in the Third Reich was a massive organization that was in charge of transporting a lot of valuable/sensitive materials. They also had some authority over lots of other forms of communication at the time, like telegraph and radio. So they required a lot of security personnel.
It looks like someone put a nail through the stock cartouche? Also appears like it was sanded a bit. Does the bolt match?