Sorry that no one apparently saw your post and answered before today... but you have basically answered your own question...
You stated that the serial numbers
don't match, and it is therefore very likely that your grip frame is one that never received a retrofit to include the holdopen as required by military order around 1916 (and remember your gun is commercial by your description so that military specs really don't apply). I would guess that any police rebuild on your pistol happened BEFORE your parts got mixed with another Luger...
This excellent illustration from Gerard Henrotin's E-book on the Luger Mechanics shows where the holdopen sits in it's recess behind and to the side of the magazine well...
While this is an inconvenience, it is hardly a reason to spend the money to have the holdopen recess machined into your frame or to swap your frame for one that has a holdopen device... of course unless you
want to
