I have a Winchester m1918 BAR that was said to have been built in March 1918, making it fairly early. The story I bought was that it was part of the Winchester Reference Library acquired by a big collector in Chicago. It has no military markings at all, which would make sense if it was taken off the line for reference purposes.
The weapon has some markings that I cannot understand: M.A.
See pics.
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My guess is that it means, "Marlin Arms" but I have nothing to back that up.