where are the all the serial numbers that people
refer too when they say a luger has matching
serial numbers

Just look at all the parts and see if they have numbers [or part of numbers] that match.

All of the Parts of a Luger that are origional to the gun when manufactured will have a matching number the Identifies it as fit to that gun. the serial number of the weapon is the master number all other parts will usually have at least the last two numbers stamped on them, this is one key factor as if the numbers do not match the gun is deminished in value and a collector will not buy it unless it is unique in some other way.