The two parallel lines are found on
all 1900 and 1902 model Lugers because they employ a toggle lock. It is where the latch for the toggle lock is fitted into the frame. You are on the right track that if the "lines" are missing it is a clue that the piece probably has been refinished. A word of caution however, I have seen original, non-refinished Lugers where these lines are barely visible and/or may not show any light color. Also, boosters can easily fake these lines by polishing away the blue with a thin instrument. This can detected under magnification, so you really need to examine a "mint" gun with a jewelers loop with at least 10X magnification. Here are a few of the examples posted by Pete Ebbink and notice in particular the top example where the right line is barely visible.
