I have made many violins, violas, and cellos. I've studied and repaired several antique violins and cellos. Most have layout lines that are still visible if you know where to look. The big time collectors think something is wrong with certain makers violins that don't show these layout marks. Either the violin was refinished or fake.
I think most of the good gun makers of the 18th century were simply making guns the way they were taught. They weren't trying to make perfect works of art; just good guns for a price that people could afford.