I think it is probably an original AE receiver with a weak chamber marking. Someone replaced the original 7.65mm barrel with a 9mm barrel and "restored/enhanced" the chamber marking with a circular punch. I do not think it is a pantograph as the line terminations are sharply squared off and do not have the rounded look of a pantograph. I am curious and would like to see a clearer photo of the mark below the serial number on the barrel if that is possible. Your photos ar pretty good for a first timer by the way
. Thank you.
Ron