Barry,
Davis's book is full of misinformation, and cannot be relied upon for diagnostic use. See this link:
http://forum.lugerforum.com/showthre...ghlight=aarron
The Alphabet Commercial Luger ("1920 Commercial") will not be found with a serial number "on the receiver" (presuming the left receiver here) in its original configuration. A gun which has been taken into Police service
may have its serial number stamped there, it would be done by the police arsenal.
Pre-WWI commercial Lugers generally have the last two digits of their serial number stamped on the recoil lug on the bottom of the receiver; sometimes this is accompanied by the previous one or two numbers stamped immediately in front of it. After the war this practice was abandoned.
--Dwight