Look at the frame where the ser# is supposed to be,,,does it look like it's been gone over with a belt grinder or a file?
Plenty of guns show up all the time with the ser# gone from a previous refinishing job.
It's not hard to tell they've been refinished at some point.
A quick measurement will also tell if the front of the frame has been taken down and the # removed.

If they look like the # has been erased,,then the # has been obliterated.
You know what that means

If it was stolen from the factory before ser#'g,,then is stolen.

Time doesn't make either one of those facts go away.

You can rationalize it away anyway you like, but reality remains.
No Lugers were manufactured with out serial numbers (or proof marks or Manufacturers markings),,That includes Military or Commercial production.