Those lines are not smooth the way you would think if they were dipped in a bluing solution. Could they be from case hardening pack not completely filling the box? If so the are not completely cased may take the bluing differently?
In the ancient past case hardening was done by packing and sealing clay jars full of the carbon bearing material with the item being hardened. The jar was expendable and broken open when the process was finished. Any reading material available on how SA did this to the Krags. It has already been said on this forum that surfaces than needed to be extra hard and wear resistant were painted with cyanide during casing, but no mention was made of the other details.

Reviewing earlier posts and Mark's mention of bluing sand fits the irregular lines very well, if it settled or the receiver was not completely filled. Maybe someone can elaborate on the process as I have never seen it described.