Why do some barrel's have the serial number and some don't.

[Ron Smith]

A good rule of thumb. If the barrel doesn't have the SN, it's a replacement, and not original to the gun. A lot of dealers will try to pass them off as "reworked".

Original "reworked" Lugers have all matching parts. The only exception would be an armorer's replacement barrel which would have the armorer's Mauser code "42" stamped on the barrel.

Ron