GRW

I could use some help with a luger I have come across. The owner tells me it is a 1920 carbine, it is a DWM, has a 12 inch barrell, checkered forearm and uncheckered buttstock. It is a 9mm, not 7.65,gun looks pretty good, bore looks real good, some of the smaller parts are mismatched. the magazine is unnumbered but wooden bottom on it. the stock has been refinished, but looks like the pictures I see of the stocks for these guns. Has grip safety and adjustable rear site. Q? How do you know it is a 1920?If anyone has any knowledge of this type of luger I would appreciate it as well as any thoughts on its value. thanks