Such straight rifling is sometimes seen in muzzleloader tubes, yes those grooves can be quite fine, and Germans have the term "Haarzüge"
I have read the idea that those straight grooves might help in reloading a badly fouled barrel. But I think nobody knows for sure what the purpose , or the hoped-for effect really was.
Remember, 1914 was not that for from muzzleloading times and superstitious recipes to improve the "Brand" or the killing properties of a barrel.
in fact such superstition is around even today, you still come across claims that a barrel "will not kill properly".