I was just looking at some pictures of a similar dated Erfurt on
http://www.phoenixinvestmentarms.com/542ErfurtPol.htm
They mention.... "A clue to the history of the gun is the milling on the front top of the receiver. The small indent at the barrel location above the date tells us the receiver was originally intended to be or was an artillery model. At some point the barrel was changed at the factory"
My Luger also has this milling. Any thoughts on the validity of that comment ?